Patent application title: STEERING SYSTEM

Abstract:

A steering system includes: an electric power steering device which
includes a steering unit of front wheels having an electric motor for
generating an auxiliary torque in accordance with at least a steering
torque and for transmitting the auxiliary torque to the steering unit;
toe angle changers for changing toe angles of rear wheels in accordance
with at least a front wheel turning angle and a vehicle speed; and a
steering controller for controlling the electric power steering device
and the toe angle changer. The steering system further includes a toe
angle changer anomaly detection unit and/or an electric power steering
device anomaly detection unit, and when a toe angle changer abnormal
state is detected, an auxiliary torque target value and/or a viscosity in
the electric power steering device is controlled, and when an electric
power steering device abnormal state is detected, the toe angle changer
is controlled.

Claims:

1. A steering system for a vehicle comprising:an electric power steering
device which comprises a steering unit of front wheels having an electric
motor configured to generate an auxiliary torque in accordance with at
least a steering torque, and is configured to transmit the auxiliary
torque to the steering unit;a toe angle changer for a left rear wheel and
a toe angle changer for a right rear wheel, which are capable of changing
toe angles of the respective left and right rear wheels in accordance
with at least a turning angle of the front wheels and a vehicle speed;
anda steering controller configured to control the electric power
steering device and the toe angle changer,wherein the steering system
further comprises at least one ofa toe angle changer anomaly detection
unit configured to detect an abnormal state of the toe angle changer,
andan electric power steering device anomaly detection unit configured to
detect an abnormal state of the electric power steering device, andwhen
an abnormal state of the toe angle changer is detected, at least one of a
target value of the auxiliary torque for assisting the electric power
steering device and a viscosity in the electric power steering device is
controlled, andwhen an abnormal state of the electric power steering
device is detected, the toe angle changer is controlled.

2. The steering system according to claim 1, whereineach of the toe angle
changers comprisesthe toe angle changer anomaly detection unit, andthe
steering controller comprisesan auxiliary torque calculating unit
configured to calculate the target value of the auxiliary torque;a yaw
rate detection unit for detecting a yaw rate of the vehicle;a yaw rate
gain calculating unit configured to calculate a yaw rate gain in the
abnormal state of the toe angle changer;a yaw rate feedback torque
correction value calculating unit configured to calculate a yaw rate
feedback torque correction value in the abnormal state of the toe angle
changer based on the yaw rate, the vehicle speed and the yaw rate gain in
the abnormal state; anda target value correction unit configured to
correct the target value of the auxiliary torque using the yaw rate
feedback torque correction value.

3. The steering system according to claim 2, wherein the yaw rate gain in
the abnormal state of the toe angle changer is calculated based on the
turning angle of the front wheels, the yaw rate and the vehicle speed.

5. The steering system according to claim 4, whereinthe steering
controller reduces the calculated target value of the auxiliary torque
when the steering controller receives the anomaly detection signal from
the toe angle changer anomaly detection unit and the vehicle speed is a
specific value or lower.

6. The steering system according to claim 4, whereinthe toe angle changers
lock both the left and right toe angles of the rear wheels in accordance
with the abnormal state, when the toe angle changer anomaly detection
unit detects an abnormal state, andthe steering controller reduces the
calculated target value of the auxiliary torque, when the steering
controller receives the anomaly detection signal from the toe angle
changer anomaly detection unit, the vehicle speed is a specific value or
lower, and at least one of the left and right toe angles of the rear
wheels locked in accordance with the abnormal state is in a toe-out
state.

8. The steering system according to claim 7, whereinthe steering
controller sets the auxiliary torque to 0 when the steering controller
receives the anomaly detection signal from the toe angle changer anomaly
detection unit and the vehicle speed is a specific value or lower.

9. The steering system according to claim 7, whereinthe toe angle changers
lock both the left and right toe angles of the rear wheels in accordance
with the abnormal state, when the toe angle changer anomaly detection
unit detects an abnormal state, andthe steering controller sets the
auxiliary torque to 0, when the steering controller receives the anomaly
detection signal from the toe angle changer anomaly detection unit, the
vehicle speed is a specific value or lower, and at least one of the left
and right toe angles of the rear wheels locked in accordance with the
abnormal is in a toe-out state.

10. The steering system according to claim 1, whereineach of the toe angle
changers comprisesa toe angle detection unit for detecting the toe angle
of the corresponding rear wheel andthe toe angle changer anomaly
detection unit,the steering controller comprisesa damping control part
configured to calculate a compensation value for increasing or reducing
the viscosity in the electric power steering device, andwhen the toe
angle changer anomaly detection unit detects an abnormal state,in the
case where the left and right rear wheels are toed in, the compensation
value is made smaller to reduce the viscosity, andin the case where the
left and right rear wheels are toed out, the compensation value is made
larger to increase the viscosity.

11. The steering system according to claim 10, whereinthe steering
controller comprisesa first table for setting the compensation value to
be referred to when the toe angle changer is in a normal state,a second
table for setting the compensation value to be referred to when the toe
angle changer is in an abnormal state and the left and right rear wheels
are toed in,a third table for setting the compensation value to be
referred to when the toe angle changer is in an abnormal state and the
left and right rear wheels are toed out, andwhen the toe angle changer
anomaly detection unit detects an abnormal state,in the case where the
left and right rear wheels are toed in, the steering controller switches
the tables to be referred to from the first table to the second table and
sets the compensation value, andin the case where the left and right rear
wheels are toed out, the steering controller switches the tables to be
referred to from the first table to the third table and sets the
compensation value.

12. The steering system according to claim 1, whereineach of the toe angle
changers comprisesa toe angle detection unit for detecting the toe angle
of the corresponding rear wheel andthe toe angle changer anomaly
detection unit,the steering controller comprisesan auxiliary torque
calculating unit configured to calculate the target value of the
auxiliary torque as an assist amount for the electric power steering
device, andwhen the toe angle changer anomaly detection unit detects an
abnormal state,in the case where the left and right rear wheels are toed
in, the target value of the auxiliary torque is made larger to make the
assist amount for the electric power steering device larger, andin the
case where the left and right rear wheels are toed out, the target value
of the auxiliary torque is made smaller to make the assist amount for the
electric power steering device smaller.

13. The steering system according to claim 12, whereinthe steering
controller comprisesa fourth table for setting the target value of the
auxiliary torque to be referred to when the toe angle changer is in a
normal state,a fifth table for setting the target value of the auxiliary
torque to be referred to when the toe angle changer is in an abnormal
state and the left and right rear wheels are toed in,a sixth table for
setting the target value of the auxiliary torque to be referred to when
the toe angle changer is in an abnormal state and the left and right rear
wheels are toed out, andwhen the toe angle changer anomaly detection unit
detects an abnormal state,in the case where the left and right rear
wheels are toed in, the steering controller switches the tables to be
referred to from the fourth table to the fifth table and sets the target
value of the auxiliary torque, andin the case where the left and right
rear wheels are toed out, the steering controller switches the tables to
be referred to from the fourth table to the sixth table and sets the
target value of the auxiliary torque.

14. The steering system according to claim 1, whereineach of the toe angle
changers comprisesa toe angle detection unit for detecting the toe angle
of the corresponding rear wheel andthe steering controller comprisesan
auxiliary torque calculating unit configured to calculate the target
value of the auxiliary torque, andthe electric power steering device
anomaly detection unit, andwhen the electric power steering device
anomaly detection unit detects an abnormal state, the toe angle changers
are controlled in a manner that at least toe-out control for allowing the
rear wheels to be toed out is not performed.

15. The steering system according to claim 14, whereinwhen the electric
power steering device anomaly detection unit detects an abnormal state
and the vehicle speed exceeds a specific value, the toe angle changers
are controlled in a manner that at least toe-out control for allowing the
rear wheels to be toed out is not performed.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application claims the foreign priority benefit under Title 35,
United States Code, section 119 (a)-(d), of Japanese Patent Application
Nos. 2007-080485, 2007-084605, 2007-088012 and 2007-104508, filed on Mar.
27, 28 and 29, and Apr. 12, 2007, respectively, in the Japan Patent
Office, the disclosures of which are herein incorporated by reference in
their entirety.

BACKGROUND OF THE INVENTION

[0002]1. Field of the Invention

[0003]The present invention relates to a steering system in which an
operation of toe angles of rear wheels is controlled based on a turning
angle of front wheels and a vehicle speed, and particularly to a steering
system in combination with an electric power steering device which
assists steering wheel turn of the front wheels.

[0004]2. Description of the Related Art

[0005]An electric power steering device is a device in which an electric
motor generates an auxiliary torque in accordance with a magnitude of a
steering torque, and the auxiliary torque is transmitted to a steering
unit, to thereby reduce a steering effort required by a driver for
steering. In Japanese patent application examined publication No.
H6-47388 (FIG. 2) (hereinbelow, referred to as "Patent Document 1"),
there is disclosed an all-wheel independent steering device in which
operation of all running wheels are individually controlled based on an
operation angle of a steering wheel and a vehicle speed. Also in Japanese
patent application unexamined specification No. 2005-199955 (FIG. 6)
(hereinbelow, referred to as "Patent Document 2"), there is disclosed a
technique in a steering device of a steer-by-wire type vehicle for
informing an anomaly to the driver, in which, when an abnormal state
(failure) associated with the steering wheel turn occurred, an electronic
control unit selects a map that gives a larger (or "heavier") steering
torque, based on a conversion table including a relationship between
turning angle and steering torque, to thereby make a steering torque in
response to the steer wheel operation of the driver become larger as
compared with a driving in a normal state.

[0006]There is also disclosed a technique in which a base signal
(auxiliary torque) based on steering torque and vehicle speed is
compensated with inertia and damping (viscosity) in the steering unit,
and an electric motor is controlled using the compensated signal as a
target current (see Japanese patent application unexamined specification
Nos. 2002-59855 (FIG. 2) and 2000-177615 (FIG. 2) (hereinbelow, referred
to as "Patent Document 3" and "Patent Document 4", respectively)).

[0007]In Patent Document 3, properties including a base signal, damping
and inertia are computed using a base table, a damper table and an
inertia table which substantially has a differential property,
respectively. Herein, setting of each table, which includes functions of
steering torque, vehicle speed and electric motor angular velocity, will
be discussed. The base table is set in such a manner that a driver is
provided with road information and a steady responsive feeling from a
steering torque, in accordance with an increase in the vehicle speed, and
thus it is required that a gain be made lower when the vehicle speed is
higher, and that a dead zone is set larger for giving a larger manual
steering zone. The base table is also set so as to give an excellent
steering feeling, and therefore, it is required that a response lag,
which may otherwise be caused by electric motor inertia, viscosity or the
like, be reduced by using the inertia table.

[0008]Since the road reaction is reduced when the vehicle runs at a higher
speed, the damping control introduces inhibitory control to the motion of
the electric motor at higher rotational speed range, to thereby impart
stability to steering feeling. Therefore, in the damping control, a
target current is corrected (damped) with a compensation value
corresponding to damping gain. In the vehicle having a toe angle changer
capable of turning the rear wheel, the toe angle of the rear wheels are
also taken into consideration upon the damping control.

[0009]In the vehicle steering system for controlling vehicle properties by
performing drive control of toe angle of rear wheels (RTC: Rear Toe
Control), drive control by RTC may be failed (drive control by RTC may be
in an abnormal state). In this case, it becomes impossible to control the
rear wheels, and the toe angle of rear wheels become fixed, leading to
change in a yaw rate gain of the vehicle. Especially, in the case of the
anomaly, such as toe-out or antiphase failure, under a high yaw rate
gain, the vehicle tends to be unstable, and thus stabilizing the vehicle
body is highly demanded.

[0010]There is disclosed a technique for stabilizing a vehicle in which
vehicle properties, such as yaw rate, are fed back to the system, and a
control with an electric power steering device is performed using a
reaction force based on the feedback (see, for example, Japanese patent
application unexamined specification No. 2003-81117 (hereinbelow,
referred to as "Patent Document 5")).

[0011]However, in the conventional techniques, such as those disclosed in
Patent Document 5 which uses the feedback of the yaw rate or the like, a
reaction force value based on the feedback is determined simply for each
vehicle speed. As a result, the reaction force value based on the
feedback, which is supposed to be determined in accordance with the
vehicle properties, cannot be properly altered. When the vehicle
properties has changed due to failure of the drive control by RTC, the
vehicle tends to be unstable, but the conventional technique cannot
respond to the change, leading to poor stabilization of the vehicle.

[0012]In addition, when the toe angle changer is in an abnormal state, for
example, when the toe angle changer is locked with the wheels toed out or
with an antiphase control state, and the vehicle runs at a higher speed,
the vehicle behavior becomes unstable.

[0013]Moreover, if the locking as described above occurs when the vehicle
runs at a low speed, the driver may not sense anomaly (may feel as
usual), due to the assisting effect by auxiliary torque in the electric
power steering device.

[0014]In order to solve this problem, it may be natural to come up with
the proposal that, when the toe angle changer is in an abnormal state,
the steering torque is made larger than usual as described in Patent
Document 2, and a means to notify the anomaly to the driver is further
provided. However, a sudden change in the steering torque in response to
the anomaly detection of the toe angle changer will give an awkward
feeling to the driver.

[0015]Also in the techniques disclosed in Patent Documents 3 and 4, when
the toe angle changer is in an abnormal state, the toe angles of rear
wheels are fixed and the reaction force properties are changed, which may
give unnatural steering feeling to the driver.

[0016]In the technique disclosed in Patent Document 1, when the power
steering device is in an abnormal state, such as no auxiliary torque is
output from the power steering device, the operational reaction force on
the steering wheel becomes larger than that in the normal state, leading
to delay in steering wheel operation by the driver. If the toe angle
changer is normally acted under such a condition, the turnability is not
reduced in response to the reduced steerability, leading to a discomfort
to the driver.

[0017]Therefore, it would be desirable to provide a steering system that
solves above-mentioned problems in vehicle stability and steerability
associated with anomaly in the relationship between the rear wheel toe
angle changer and the power steering, by performing a control in the
combination of the power steering and the rear wheel toe angle changer.

SUMMARY OF THE INVENTION

[0018]In one aspect of the present invention, there is provided a steering
system for a vehicle including: an electric power steering device which
includes a steering unit of front wheels having an electric motor
configured to generate an auxiliary torque in accordance with at least a
steering torque, and is configured to transmit the auxiliary torque to
the steering unit; a toe angle changer for a left rear wheel and a toe
angle changer for a right rear wheel, which are capable of changing toe
angles of the respective right and left rear wheels in accordance with at
least a turning angle of the front wheels and a vehicle speed; and a
steering controller configured to control the electric power steering
device and the toe angle changer, wherein the steering system further
includes at least one of a toe angle changer anomaly detection unit
configured to detect an abnormal state of the toe angle changer, and an
electric power steering device anomaly detection unit configured to
detect an abnormal state of the electric power steering device, and when
an abnormal state of the toe angle changer is detected, at least one of a
target value of the auxiliary torque for assisting the electric power
steering device and a viscosity in the electric power steering device is
controlled, and when an abnormal state of the electric power steering
device is detected, the toe angle changer is controlled.

[0019]In the above-mentioned steering system, it is preferable that each
of the toe angle changers includes the toe angle changer anomaly
detection unit, and the steering controller includes an auxiliary torque
calculating unit configured to calculate the target value of the
auxiliary torque; a yaw rate detection unit for detecting a yaw rate of
the vehicle; a yaw rate gain calculating unit configured to calculate a
yaw rate gain in the abnormal state of the toe angle changer; a yaw rate
feedback torque correction value calculating unit configured to calculate
a yaw rate feedback torque correction value in the abnormal state of the
toe angle changer based on the yaw rate, the vehicle speed and the yaw
rate gain in the abnormal state; and a target value correction unit
configured to correct the target value of the auxiliary torque using the
yaw rate feedback torque correction value.

[0020]In the above-mentioned steering system, it is preferable that each
of the toe angle changers includes the toe angle changer anomaly
detection unit, the steering controller includes an auxiliary torque
calculating unit configured to calculate the target value of the
auxiliary torque, and the steering controller reduces the calculated
target value of the auxiliary torque when the steering controller
receives an anomaly detection signal from the toe angle changer anomaly
detection unit.

[0021]In the above-mentioned steering system, it is preferable that each
of the toe angle changers includes the toe angle changer anomaly
detection unit, and the steering controller sets the auxiliary torque to
0 when the steering controller receives an anomaly detection signal from
the toe angle changer anomaly detection unit.

[0022]In the above-mentioned steering system, it is preferable that each
of the toe angle changers includes a toe angle detection unit for
detecting the toe angle of the corresponding rear wheel and the toe angle
changer anomaly detection unit, the steering controller includes a
damping control part configured to calculate a compensation value for
increasing or reducing the viscosity in the electric power steering
device, and when the toe angle changer anomaly detection unit detects an
abnormal state, in the case where the left and right rear wheels are toed
in, the compensation value is made smaller to reduce the viscosity, and
in the case where the left and right rear wheels are toed out, the
compensation value is made larger to increase the viscosity.

[0023]In the above-mentioned steering system, it is preferable that each
of the toe angle changers includes a toe angle detection units for
detecting the toe angle of the corresponding rear wheel and the toe angle
changer anomaly detection unit, the steering controller includes an
auxiliary torque calculating unit configured to calculate the target
value of the auxiliary torque as an assist amount for the electric power
steering device, and when the toe angle changer anomaly detection unit
detects an abnormal state, in the case where the left and right rear
wheels are toed in, the target value of the auxiliary torque is made
larger to make the assist amount for the electric power steering device
larger, and in the case where the left and right rear wheels are toed
out, the target value of the auxiliary torque is made smaller to make the
assist amount for the electric power steering device smaller.

[0024]In the above-mentioned steering system, it is preferable that each
of the toe angle changers includes a toe angle detection units for
detecting the toe angle of the corresponding rear wheel and the steering
controller includes an auxiliary torque calculating unit configured to
calculate the target value of the auxiliary torque, and the electric
power steering device anomaly detection unit, and when the electric power
steering device anomaly detection unit detects an abnormal state, the toe
angle changers are controlled in a manner that at least toe-out control
for allowing the rear wheels to be toed out is not performed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]The various aspects, other advantages and further features of the
present invention will become more apparent by describing in detail
illustrative, non-limiting embodiments thereof with reference to the
accompanying drawings.

[0026]FIG. 1 is a schematic diagram of a four-wheel vehicle having a
steering system according to a first embodiment of the present invention.

[0027]FIG. 2 is a diagram of an electric power steering device in the
steering system.

[0028]FIG. 3 is a schematic plan view of a toe angle changer on a left
rear wheel side in the steering system.

[0029]FIG. 4 is a schematic cross sectional view showing a structure of an
actuator of a toe angle changer.

[0030]FIG. 5 is a schematic diagram of a control function of a steering
control ECU and toe angle changers in the steering system according to
the first embodiment.

[0031]FIG. 6A is a graph showing a function of base signal stored in a
base table, FIG. 6B is a graph showing a characteristic function of a
damper table, and FIG. 6C is a graph showing relationships among a damper
table N, a damper table A and a damper table B.

[0034]FIG. 9 is a block configuration diagram showing a control function
of a toe angle change control ECU of a toe angle changer.

[0035]FIG. 10 is a schematic diagram of a four-wheel vehicle having a
steering system according to a second embodiment of the present
invention.

[0036]FIG. 11 is a diagram of an electric power steering device in the
steering system.

[0037]FIG. 12 is a schematic cross sectional view showing a structure of
an actuator of a toe angle changer.

[0038]FIG. 13 is a schematic diagram of a control function of a steering
control ECU and toe angle changers in the steering system according to
the second embodiment.

[0039]FIG. 14 is a graph illustrating an output signal IM2 relative
to an input signal IM1, when a correction command signal is received
in an anomaly auxiliary torque restriction part.

[0040]FIG. 15 is a block configuration diagram showing a control function
of a toe angle change control ECU of a toe angle changer.

[0041]FIG. 16 is a flow chart showing a control in a toe angle change
control diagnostic part.

[0042]FIG. 17 is a schematic diagram of a control function of a steering
control ECU and toe angle changers in the steering system according to a
modified version of the second embodiment.

[0043]FIG. 18 is a schematic diagram of a four-wheel vehicle having a
steering system according to a third embodiment of the present invention.

[0044]FIG. 19 is a diagram of an electric power steering device in the
steering system.

[0045]FIG. 20 is a schematic diagram of a control function of a steering
control ECU and toe angle changers in the steering system according to
the third embodiment.

[0046]FIG. 21A is a graph showing relationships between turning angle and
yaw rate for different toe angles of rear wheels, when a vehicle runs on
a slalom road, and FIG. 21B is a graph showing relationships between
turning angle and steering torque for different toe angles of rear
wheels, when a vehicle runs on a slalom road.

[0047]FIG. 22 is a flow chart showing a control in which a steering
control ECU detects an anomaly of the toe angle changer and a
compensation value for damping control is set.

[0048]FIG. 23 is a schematic diagram of a control function of a steering
control ECU and toe angle changers in the steering system according to a
fourth embodiment.

[0049]FIG. 24 is a flow chart showing a control in which a steering
control ECU detects an anomaly of the toe angle changer and a base signal
is set.

[0050]FIG. 25 is a schematic diagram of a four-wheel vehicle having a
steering system according to a fifth embodiment of the present invention.

[0051]FIG. 26 is a schematic diagram of a control function of a steering
control ECU and toe angle changers in the steering system according to a
fifth embodiment.

[0052]FIG. 27 is a flow chart showing a control for changing toe angle of
the rear wheels when an electric power steering device has an anomaly in
auxiliary torque.

[0053]FIG. 28 is a schematic diagram of a control function of a steering
control ECU and toe angle changers in the steering system according to a
first modified version of the fifth embodiment.

[0054]FIG. 29 is a detailed diagram of a control function of a target toe
angle computing part of the steering control ECU.

[0055]FIG. 30 show a comparison in yaw rate response property in a turning
motion between a standard vehicle model according to the first modified
version of the fifth embodiment and a conventional vehicle, in which:
FIG. 30A is a graph showing a gain property of yaw rate, and FIG. 30B is
a graph showing a phase lag property of yaw rate.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0056]Embodiments of the present invention will be described in detail
below with reference to the accompanying drawings.

First Embodiment

1. Configuration of Steering System

[0057]As shown in FIG. 1, a steering system 100 includes an electric power
steering device 110 having an electric motor 4, which is configured to
assist steering of front wheels 1L, 1R by a steering wheel 3; toe angle
changers 120L, 120R configured to independently change respective toe
angles of rear wheels 2L, 2R by respective actuators 30, in accordance
with a turning angle of the front wheels 1L, 1R by the electric power
steering device 110 and a vehicle speed; a steering controller
(hereinbelow, referred to as "steering control ECU (Electronic Control
Unit)") 130 configured to control the electric power steering device 110
and the toe angle changers 120L, 120R; and various sensors, including a
vehicle speed sensor SV, a yaw rate sensor SY (yaw rate
detection unit) and a lateral acceleration sensor SGS.

2. Configuration of Electric Power Steering Device

[0058]The electric power steering device 110 includes, as shown in FIG. 2,
the steering wheel 3, a main steering shaft 3a attached thereto, a shaft
3c and a pinion shaft 7, which shafts are connected through two universal
joints 3b. The pinion shaft 7 has a pinion gear 7a provided on a lower
end of the pinion shaft 7, which engages with rack teeth 8a of a rack
shaft 8 which can reciprocate in a vehicle width direction. To respective
ends of the rack shaft 8, the left front wheel 1L and the right front
wheel 1R are connected through tie rods 9, 9. With this configuration,
the electric power steering device 110 can change traveling direction of
the vehicle by the operation of the steering wheel 3. Herein, the rack
shaft 8, the rack teeth 8a and the tie rods 9, 9 constitute a steering
wheel turn mechanism.

[0059]It should be noted that the pinion shaft 7 is supported by a
steering gear box 6: an upper portion, a middle portion and a lower
portion of the pinion shaft 7 are supported through bearings 3d, 3e and
3f, respectively.

[0060]The electric power steering device 110 also has the electric motor 4
for supplying an auxiliary steering effort (auxiliary torque) to reduce a
steering effort (steering torque) required at the steering wheel 3. The
electric motor 4 has an output shaft with a worm gear 5a which engages
with a worm wheel gear 5b provided on the pinion shaft 7.

[0061]In other words, the worm gear 5a and the worm wheel gear 5b
constitute a deceleration mechanism. In addition, a rotor (not shown) of
the electric motor 4, and the components connected to the electric motor
4, such as the worm gear 5a, the worm wheel gear 5b, the pinion shaft 7,
the rack shaft 8, the rack teeth 8a and the tie rods 9, 9, constitute a
steering unit.

[0062]The electric motor 4 is a three-phase brushless motor formed of a
stator (not shown) with a plurality of field coils as well as the rotor
which rotates in the stator, for converting electric energy to kinetic
energy (PM=ωTM).

[0063]Herein, ω represents a rotational angular velocity
(hereinbelow, also simply referred to as "angular velocity") of the
electric motor 4, and TM represents a torque generated at the
electric motor 4. In addition, a relationship between the generated
torque TM and an output torque TM* actually obtained as an
output can be represented by the following formula (1):

TM*=TM-(Cmdθm/dt+Jmd2θm/dt.-
sup.2)i2 (1)

[0064]where i represents a reduction gear ratio of the worm gear 5a to the
worm wheel gear 5b; θm represents the rotation angle of the
electric motor; and Jm and Cm represent the inertia moment and
the viscosity coefficient, respectively, of the rotor of the electric
motor 4.

[0065]As is apparent from the formula (1), the relationship between
TM* and θm can be expressed with Jm and Cm of
the rotor of the electric motor 4, which means the relationship is
independent of the vehicle properties or the vehicle state.

[0066]Herein a steering torque applied to the steering wheel 3 is
represented as TS, and a coefficient of an assist amount AH by
the torque (auxiliary torque) generated at the electric motor 4, which
has been powered through the deceleration mechanism, is represented as,
for example, kA(V), which varies as a function of the vehicle speed
V. Since the formula AH=kA(V)×TS is established in
this case, a pinion torque TP, which is a road load, can be
represented by the following formula (2):

T P = T S + A H = T S + k A ( V ) ×
T S ( 2 )

[0067]From this formula, the steering torque TS can be represented by
the following formula (3).

TS=TP/(1+kA(V)) (3)

[0068]Therefore, the steering torque TS is reduced to
1/(1+kA(V)) of the pinion torque TP (load). For example, if
kA(0)=2 with the vehicle speed V=0 km/h, the steering torque TS
is controlled to one third of the pinion torque TP, and if
kA(100)=0 with the vehicle speed V=100 km/h, the steering torque
TS is controlled to be equal to the pinion torque TP, which
provides a responsive feeling from a steady steering torque, similar to
those obtained in the manual steering. In other words, by controlling the
steering torque TS in accordance with the vehicle speed V, the
responsive feeling from the steering torque becomes light when the
vehicle runs at a lower speed, and steady and stable when the vehicle
runs at a higher speed.

[0069]In general, the steering torque TS is known to be represented
by the following formula (4):

TS=Jd2θS/dt2+CdθS/dt+K(θS--
θF) (4)

[0070]where θs represents a steering rotation angle;
θF represents a value obtained by dividing an electric motor
rotation angle θm by a rotation ratio nM of the
deceleration mechanism; J represents an inertia coefficient (inertia) of
a steering unit; C represents a viscosity coefficient (damper) of the
steering unit; and K represents a base signal coefficient. Like in the
formula (1), the relationship shown by the above formula is independent
of the vehicle properties or the vehicle state.

[0071]Upon evaluating the steering feeling, attention is paid upon a
difference in the steering reaction force due to the presence or absence
of the inertia moment, and the inertia moment can be evaluated with a
ratio Ev of an input torque to a reaction force torque (inertia torque)
taken as a performance function. Based on the evaluation result using
this performance function, it was found that there is a zone with a high
steering feeling evaluation near a point Ev0, where the inertia
coefficient J in the steering unit and the viscosity around the steering
unit are neglected.

[0072]Herein, the ratio Ev is represented by the following formula:

Ev=Tdet/TS=K(θS-θF)/TS (5)

[0073]With this formula, a compensator H(S) can be obtained which imparts
the gain property of the transfer function from TS to Tdet=K
(θS-θF) retained to Ev0 or less.

[0074]Since the compensator obtained by the H∞ control is determined
by a subject model and an order (degree) of the frequency weighing
function, the compensator will be with a higher order which is difficult
to obtain with regular microcomputer. Therefore, the order of the
compensator is reduced here by focusing on the frequency band required
for controlling the steering unit.

[0075]For example, the transfer function Hf(S) can be set to the following
formula so as to have four zeros and four poles:

[0076]It should be noted that the gain property of the transfer function
Hf(S) has a differentiation property.

[0077]In addition, the electric power steering device 110 also includes an
electric motor drive circuit 23 configured to drive the electric motor 4;
a resolver 25; a torque sensor ST configured to detect (measure) a
pinion torque TP applied to the pinion shaft 7; a differential
amplifier 21 configured to amplify the output from the torque sensor
ST; and the vehicle speed sensor SV configured to detect
(measure) a vehicle speed.

[0078]As shown in FIG. 5, the steering control ECU 130 of the steering
system 100 has an electric power steering control part 130a (which will
be described later) as a functional part of the electric power steering
device 110, which controls the driving of the electric motor 4, as well
as a rear wheel toe angle control part 130b.

[0079]The electric motor drive circuit 23 has switching elements, such as
three-phase FET bridge circuit, and is configured to generate a
square-wave voltage based on duty signals (DU, DV, DW) from the electric
power steering control part 130a (see FIG. 5), to thereby drive the
electric motor 4.

[0081]The resolver 25 is configured to detect (measure) a rotation angle
θm of the electric motor 4 and to output an angular signal
θ, and examples include a sensor for detecting a change in
magnetoresistance which is positioned in the vicinity of a magnetic rotor
having a plurality of recess portions and projection portions arranged
evenly along a circumference of the rotor.

[0082]The torque sensor ST is configured to detect (measure) the
pinion torque TP applied to the pinion shaft 7. The torque sensor
ST is formed of magnetostrictive films adhered to the pinion shaft 7
at two different positions along an axis thereof so as to exhibit
opposite anisotropies, and detection coils are arranged with a gap from
the pinion shaft 7 along the surface (outer circumference) of the
respective magnetostrictive films.

[0083]The differential amplifier 21 is configured to amplify a difference
in permeability change between two magnetostrictive films detected as an
inductance change by the detection coil, and to output a torque signal T.

[0084]The vehicle speed sensor SV is configured to detect (measure)
the vehicle speed V as a pulse number per unit time, and to output a
vehicle speed signal VS.

[0085]The functional configuration of the steering control ECU 130 will be
described later, together with the control by the electric power steering
device 110 and the control by the toe angle changers 120L, 120R.

3. Configuration of Toe Angle Changer

[0086]Next, a configuration of the toe angle changer will be described
with reference to FIGS. 3 and 4.

[0087]FIG. 3 is a schematic plan view of a toe angle changer on a left
rear wheel side. FIG. 4 is a schematic cross sectional view showing a
structure of an actuator of a toe angle changer.

[0088]The toe angle changers 120L, 120R are installed to the left rear
wheel 2L and the right rear wheel 2R of the vehicle, respectively. The
toe angle changer 120L is taken as an example, and the left rear wheel 2L
is shown in FIG. 3. The toe angle changer 120L includes the actuator 30
and a toe angle change controller (hereinbelow, referred to as "toe angle
change control ECU") 37.

[0089]It should be noted that FIG. 3 shows only the left rear wheel 2L,
but the components are arranged in the same manner (symmetrically) on the
right rear wheel 2R.

[0090]The cross member 12 extends substantially in the vehicle width
direction, and end portions (in terms of the vehicle width direction)
thereof are elastically supported by a rear side frame 11 of the vehicle
body. A trailing arm 13 extends substantially in the front-rear direction
of the vehicle body, and a front end portion thereof is supported by a
portion near the terminal (in terms of the vehicle width direction) of
the cross member 12. The rear wheel 2L is fixed to a rear end portion of
the trailing arm 13.

[0091]The trailing arm 13 is formed of a vehicle body-side arm 13a
attached to the cross member 12, and a wheel-side arm 13b fixed to the
rear wheel 2L, which are connected to each other through a nearly
vertical rotation axis 13c. With this configuration, the trailing arm 13
is displaceable in the vehicle width direction.

[0092]With respect to the actuator 30, one end portion is attached through
a ball joint 16 to a front end portion of the wheel-side arm 13b relative
to the rotation axis 13c, and the other end (base end) portion of the
actuator 30 is fixed to the cross member 12 through a ball joint 17.

[0093]As shown in FIG. 4, the actuator 30 includes an electric motor 31, a
deceleration mechanism 33, a feed screw portion 35 and the like.

[0094]The electric motor 31 may be a brush motor, a brushless motor or the
like, which can rotate in both forward and reverse directions.

[0095]The feed screw portion 35 includes: a rod 35a in a shape of a
cylinder; a nut 35c in a shape of a cylinder which has an internal thread
35b formed in an inner periphery thereof and is inserted in the rod 35a;
and a screw shaft 35d which engages with the internal thread 35b and
supports the rod 35a in such a manner that the rod 35a is movable in an
axial direction.

[0096]The feed screw portion 35, the deceleration mechanism 33 and the
electric motor 31 are encased in a case body 34 in an elongated
cylindrical shape. To a portion of the case body 34 on a feed screw
portion 35 side, a boot 36 is attached so as to cover both an end portion
of the case body 34 and an end portion of the rod 35a, in order to
prevent dust or foreign matter from attaching to an outer periphery of
the rod 35a exposed from the end portion of the case body 34, and to
prevent dust, foreign matter or water from entering the case body 34.

[0097]One end portion of the deceleration mechanism 33 is connected to an
output shaft of the electric motor 31, and the other end portion is
connected to the screw shaft 35d.

[0098]When the power of the electric motor 31 is transmitted through the
deceleration mechanism 33 to the screw shaft 35d to rotate the screw
shaft 35d, the rod 35a shifts in a right-left direction in the drawing
(axial direction) relative to the case body 34, and thus the actuator 30
contracts or expands. Due to the frictional force caused by engagement of
the screw shaft 35d and the internal thread 35b of the nut 35c, a toe
angle of the rear wheel is maintained constant, even when the electric
motor 31 is not energized and not driven.

[0099]The actuator 30 also includes a stroke sensor 38 configured to
detect (measure) the position of the rod 35a (i.e., amount of
expansion/contraction). In the stroke sensor 38, a magnet or the like is
embedded so as to detect (measure) the location of the rod 35a by
utilizing magnetism. In this manner, by detecting the position of the rod
35a using the stroke sensor 38, the steering angles (toe angle) of toe-in
or toe-out of the rear wheels 2L, 2R are separately detected with high
accuracy.

[0100]With the actuator 30 having the configuration as described above,
the ball joint 16 provided on an end portion of the rod 35a is rotatably
connected to the wheel-side arm 13b of the trailing arm 13 (see FIG. 3),
and the ball joint 17 provided on the base end of the case body 34
(right-hand end in FIG. 4) is rotatably connected to the cross member 12
(see FIG. 3). When the power of the electric motor 31 rotates the screw
shaft 35d and the rod 35a shifts leftward (in FIG. 4) (i.e., the actuator
30 expands), the wheel-side arm 13b is pushed outward in the vehicle
width direction (left direction in FIG. 3) to thereby leftward turn the
rear wheel 2L. On the other hand, when the rod 35a shift rightward (in
FIG. 4) (i.e. the actuator 30 contracts), the wheel-side arm 13b is
pulled inward in the vehicle width direction (right direction in FIG. 3)
to thereby rightward turn the rear wheel 2L.

[0101]It should be noted that the position to which the ball joint 16 of
the actuator 30 is attached is not limited to the wheel-side arm 13b and
the actuator 30 can be attached to any position, such as on a knuckle
arm, as long as the toe angle of the rear wheel 2L can be changed. In
addition, in the present embodiment, the toe angle changers 120L, 120R
are applied to an independent suspension system with semi-trailing arms.
However, the present invention is not limited to this type of suspension
system, and may be applied to other types of suspension system.

[0102]For example, the actuator 30 may be introduced to a side rod of a
double wishbone type suspension, or a side rod of a strut type
suspension.

[0103]In addition, the toe angle change control ECU 37 is unified with the
actuator 30. The toe angle change control ECU 37 is fixed to the case
body 34 of the actuator 30, and connected to the stroke sensor 38 through
connectors or the like. Between two toe angle change control ECUs 37, 37,
and between the toe angle change control ECU 37 and the steering control
ECU 130, there are provided signal circuits connecting them to each
other.

[0104]To the toe angle change control ECU 37, power is supplied from a
power source (not shown), such as a battery, mounted on a vehicle. Also
to the steering control ECU 130 and the electric motor drive circuit 23,
power is supplied from a power source (not shown), such as battery, which
is an independent system of that of the toe angle change control ECU 37.

4. Functional Configuration of Steering Control ECU

[0105]Next, functions of the steering control ECU will be described with
reference to FIGS. 5, 6A, 6B, 7, 8A and 8B.

[0106]FIG. 5 is a schematic diagram of a control function of a steering
control ECU and toe angle changers in the steering system, FIG. 6A is a
graph showing a function of base signal stored in a base table, FIG. 6B
is a graph showing a characteristic function of a damper table, FIG. 7 is
a block configuration diagram showing detailed functions of a yaw rate
feedback torque correction computing part shown in FIG. 5, and FIGS. 8A
and 8B show characteristics of the yaw rate feedback torque correction
computing part.

[0107]The steering control ECU 130 includes a microcomputer with
components, such as CPU (Central Processing Unit), ROM (Read Only
Memory), RAM (Random Access Memory) (all not shown), programs and
peripheral circuits and the like, and performs a function depicted in the
control function diagram of FIG. 5.

[0108]As shown in FIG. 5, the steering control ECU 130 includes: the
electric power steering control part 130a configured to control the
electric power steering device 110; and a rear wheel toe angle control
part 130b configured to compute (control) toe angles of the rear wheel
2L, 2R. The steering control ECU 130 is connected to the electric motor
drive circuit 23 of the electric power steering device 110 (see FIG. 2),
the left toe angle changer 120L and the right toe angle changer 120R and
configured to control these components. The steering control ECU 130 is
also connected to various sensors and configured to receive variety of
information from the sensors.

(Electric Power Steering Control Part)

[0109]First, the electric power steering control part 130a will be
described with reference to FIGS. 5, 6A and 6B (and FIG. 2 where
appropriate).

[0110]The electric power steering control part 130a includes: a base
signal computing part (auxiliary torque calculating unit) 51; a damper
compensation signal computing part (auxiliary torque calculating unit)
52; an inertia compensation signal computing part (auxiliary torque
calculating unit) 53; a Q-axis (torque axis) PI control part 54; a D-axis
(axis of magnetic pole) PI control part 55; a 2-axis-to-3-phase
conversion part 56; a PWM conversion part 57; a 3-phase-to-2-axis
conversion part 58; an electric motor speed calculating part 67; and an
exciting current generation part 59.

[0111]The 3-phase-to-2-axis conversion part 58 converts a three-phase
current IU, IV, IW of the electric motor 4 detected by the electric motor
drive circuit 23 into a two-axis current, including a D-axis which is an
axis of magnetic pole of the rotor of the electric motor 4, and a Q-axis
which is obtained by electrically rotating the D-axis by 90 degrees. A
Q-axis current IQ is proportional to the torque TM generated at the
electric motor 4, and a D-axis current ID is proportional to an exciting
current. The electric motor speed calculating part 67 introduces a
differential operator to an angular signal θ of the electric motor
4, to thereby generate an angular velocity signal ω. The exciting
current generation part 59 generates a target signal for the exciting
current of the electric motor 4, and if desired, field-weakening control
can be performed by making the D-axis current substantially equal to the
Q-axis current.

[0112]Based on the torque signal T and the vehicle speed signal VS, the
base signal computing part 51 generates a base signal DT to be used
as a standard reference for a target signal IM1 of the output torque
TM*. The signal is generated from a base table 51a with reference to
the torque signal T and the vehicle speed signal VS, which table had been
prepared by experimental measurement or the like. FIG. 6A is a graph
showing a function of the base signal DT, stored in the base table
51a. In the base signal computing part 51, a dead zone N1 is provided
where the base signal DT is set to zero when the value of the torque
signal T is small, and the base signal DT linearly increases along a
gain G1 when the value of the torque signal T is larger than the value in
the dead zone N1. The base signal computing part 51 increases the output
along a gain G2 at specific torque values, and when the torque value
further increases, the output is made saturated.

[0113]In addition, a vehicle body in general has various road loads (road
reactions) depending on the running speed thereof. Accordingly, the gain
is adjusted based on the vehicle speed signal VS. The load is heaviest
during a static steering (vehicle speed=0), and the load is relatively
small at medium and low speeds. Therefore, when the vehicle speed V
becomes higher, the base signal computing part 51 provides the driver
with road information with a larger manual steering zone, by making the
gains (G1, G2) smaller and the dead zone N1 larger. In other words, in
accordance with the increase of the vehicle speed V, a steady responsive
feeling is provided from the steering torque TS. In this case, it is
necessary that the inertia compensation be made also in the manual
steering zone.

[0114]Referring to FIG. 5, the damper compensation signal computing part
52 is introduced for compensating a viscosity in the steering unit, and
for providing a steering damper function for compensating convergence of
steering wheel position when convergence decreases during high-speed
driving, by reading a damper table 52a with reference to the angular
velocity signal ω.

[0115]FIG. 6B is a graph showing a characteristic function of the damper
table 52a, in which the line is formed of a several linear sections and a
compensation value I as a whole increases as the rotational angular
velocity ω of the electric motor 4 (see FIG. 2) increases. The
graph is also characterized in that the compensation value I rapidly
increases when the rotational angular velocity ω is in a specific
range. Moreover, as the vehicle speed signal VS becomes high, i.e. the
vehicle speed V becomes high, the rotational angular velocity ω of
the electric motor 4, i.e., the output torque TM* of the electric
motor 4 in accordance with the speed of the steering wheel turn, is
reduced by increasing the gains (G3, G5). To put it another way, when the
steering wheel 3 is turned away from the home position, a current to the
electric motor 4 is reduced; when the steering wheel 3 is returned to
resume the home position, a large current is supplied to the electric
motor 4. For example, when the steering wheel 3 is further turned away
and the rotational angular velocity ω becomes high, the rotational
angular velocity ω cannot be immediately reduced because of the
inertia in the electric motor 4. In order to prevent this phenomenon, the
damper compensation signal computing part 52 increases the current supply
to the electric motor 4, to thereby perform an inhibitory control of the
rotational angular velocity ω when the steering wheel 3 is resuming
the home position. Because of this steering damper effect, convergence of
the steering wheel 3 is improved, to thereby stabilize the vehicle
properties.

[0116]Referring to FIG. 5, an adder 61 is configured to subtract the
output signal (compensation value I) of the damper compensation signal
computing part 52 from the output signal DT of the base signal
computing part 51, and an adder 62 is configured to add the output signal
from the adder 61 and the output signal from the inertia compensation
signal computing part 53 and to output the output signal IM1.

[0117]It should be noted that an assist control is performed by a
combination of the base signal computing part 51, the damper compensation
signal computing part 52 and the adder 61.

[0118]The inertia compensation signal computing part 53 is configured to
compensate an effect caused by the inertia in the steering unit, in which
the torque signal T is computed from an inertia table 53a. A transfer
function Hf(S) in the stored table is, for example, represented by the
following formula:

[0119]In addition, the inertia compensation signal computing part 53
compensates the lowering of the response caused by the inertia of the
rotor of the electric motor 4. To put it another way, when the rotation
direction of the electric motor 4 is made to be switched from forward to
reverse or vice versa, it is difficult to immediately switch the
direction since the inertia tends to maintain the rotational state.
Accordingly, the inertia compensation signal computing part 53 controls
the timing of switching the rotation direction of the electric motor 4,
so as to synchronize the timing of switching the rotation direction of
the electric motor 4 with that of the steering wheel 3. In this manner,
the inertia compensation signal computing part 53 reduces a response lag
in the steering, which may otherwise be caused by inertia, viscosity or
the like in the steering unit, to thereby give an excellent steering
feeling.

[0121]The output signal IM1 of the adder 62 is a target signal for
the Q-axis current which defines the torque of the electric motor 4 (see
FIG. 2).

[0122]An adder 63 (target value correction unit) is configured to
subtract, from the output signal IM1, a yaw rate feedback torque
correction value IY (which may be, hereinbelow, simply referred to
as "correction value IY") output from a yaw rate feedback torque
correction computing part 72, which is calculated based on the yaw rate
γ and the vehicle speed V, as well as a yaw rate gain set by the
present turning angle of front wheel (which is also referred to as "tire
angle") δ, the yaw rate γ or the like, and to send an output
signal IM2 to an adder 64.

[0123]The details of the yaw rate feedback torque correction computing
part 72 will be described later.

[0124]The adder 64 is configured to subtract the Q-axis current IQ from
the output signal IM2, and to generate a deviation signal IE. The
Q-axis (torque axis) PI control part 54 is configured to perform a P
(proportional) control and an I (integral) control so as to reduce the
deviation signal IE.

[0125]An adder 65 is configured to subtract the D-axis current ID from the
output signal of the exciting current generation part 59. The D-axis
(axis of magnetic pole) PI control part 55 is configured to perform a PI
feedback control so as to reduce the output signal from the adder 65.

[0126]The 2-axis-to-3-phase conversion part 56 is configured to convert
two-axis signal including an output signal VQ from the Q-axis (torque
axis) PI control part 54 and an output signal VD from the D-axis (axis of
magnetic pole) PI control part 55 into three-phase signal UU, UV, UW. The
PWM conversion part 57 is configured to generate duty signals (DU, DV,
DW), which is an ON/OFF signal [PWM (Pulse Width Modulation) signal]
having pulse widths proportional to the magnitude of the three-phase
signal UU, UV, UW.

[0127]It should be noted that the angular signal θ of the electric
motor 4 (see FIG. 2) is input to the 2-axis-to-3-phase conversion part 56
and the PWM conversion part 57, and a signal corresponding to the
magnetic pole position of the rotor is output.

[0128](Rear Wheel Toe Angle Control Part)

[0129]Next, the rear wheel toe angle control part 130b will be described
with reference to FIGS. 5, 7, 8A and 8B. As shown in FIG. 5, the rear
wheel toe angle control part 130b includes a front wheel turning angle
computing part 68, a target toe angle computing part 71, the yaw rate
feedback torque correction computing part 72 and a toe angle change
control diagnostic part 73.

[0130]The front wheel turning angle computing part 68 is configured to
calculate a turning angle δ of the front wheels 1L, 1R based on the
angular signal θ output from the resolver 25, and to input the
result to the target toe angle computing part 71 and the yaw rate
feedback torque correction computing part 72.

[0131]The target toe angle computing part 71 is configured to generate
target values of toe angles for the rear wheels 2L, 2R, respectively,
based on the vehicle speed signal VS, a turning angle δ of the
front wheels 1L, 1R, and the rotational angular velocity ω of the
electric motor 4 which is proportional to the turning angular velocity of
the front wheels 1L, 1R. The target values are generated from a rear
wheel toe angle table 71a, with reference to the turning angle δ,
the vehicle speed signal VS, and the rotational angular velocity ω,
which table had been prepared for each of the left rear wheel 2L and the
right rear wheel 2R in advance.

[0132]When the vehicle speed V is in a specific low-speed range, each of
the target values for the toe angle of the rear wheels is generated in
such a manner that the rear wheels 2L, 2R are in antiphase relative to
the front wheels, in accordance with the turning angle δ, to allow
the vehicle to turn in a small radius.

[0133]In the high-speed range over the above-mentioned specific low-speed
range, when an absolute value of the rotational angular velocity ω
is a specific value or less, and at the same time, the turning angle
δ is within a specific range (including right and left), each of
the target values of the toe angles of the rear wheels 2L, 2R are set as
the same phase relative to the front wheels, in accordance with the
turning angle δ. In other words, each of the target values of the
toe angles of the rear wheels 2L, 2R are set so as to make the slip angle
β small during lane change.

[0134]However, in the high-speed range over the above-mentioned specific
low-speed range, when the absolute value of the rotational angular
velocity ω exceeds a specific value, or when the turning angle
δ is too large to fall outside the specific range (including right
and left), each of the target values of the toe angles of the rear wheels
2L, 2R is set to antiphase relative to the front wheels, in accordance
with the turning angle δ. The target value of the toe angle
generated in the target toe angle computing part 71 is input to the yaw
rate feedback torque correction computing part 72 and the respective toe
angle change control ECUs 37, 37 of the toe angle changers 120L, 120R.

[0135]It should be noted that, the target values of the toe angles
generated in the target toe angle computing part 71 follow
Ackerman-Jeantaud geometry.

[0136]The detailed configuration of the yaw rate feedback torque
correction computing part 72 will be described with reference to FIG. 7.
The yaw rate feedback torque correction computing part 72 has a yaw rate
gain calculating part 72a, a yaw rate feedback torque correction value
calculating part 72b and a yaw rate feedback map 72c.

[0137]The yaw rate gain calculating part 72a is configured to calculate a
yaw rate gain GY in the case where the toe angle changers 120R
and/or 120L are in an abnormal state, based on the vehicle speed V, the
yaw rate γ, the turning angle δ of the front wheels, and an
anomaly signal E received from the toe angle change control diagnostic
part 73, which will be described later. The calculation of the yaw rate
gain GY is initiated when the anomaly signal E is received and thus
it is determined that the toe angle changers 120R and/or 120L are in an
abnormal state, and the calculation is made based on the vehicle speed V,
the yaw rate γ and the turning angle δ of the front wheels.
In other words, in order to obtain a target yaw rate in an abnormal
state, the yaw rate gain calculating part 72a calculates the yaw rate
gain GY from the relationship between the turning angle δ of
the front wheels and the yaw rate γ, by altering the vehicle speed
V as needed. The calculated yaw rate gain GY is output to the yaw
rate feedback torque correction value calculating part 72b, and used for
calculating a correction value IY, which will be described later.
The calculated yaw rate gain GY is output also to the yaw rate
feedback map 72c, and is used for generating and updating the map.

[0138]The yaw rate feedback torque correction value calculating part 72b
is configured to calculate a yaw rate feedback torque correction value
IY, which acts as a reaction force to the assisting force by the
electric power steering device 110, based on the vehicle speed V, the yaw
rate γ, the yaw rate gain GY and the anomaly signal E. The
calculation of the yaw rate feedback torque correction value IY is
initiated when the anomaly signal E is received and thus it is determined
that the toe angle changers 120R and/or 120L are in an abnormal state,
and the calculation is made based on the vehicle speed V, the yaw rate
γ and the yaw rate gain GY. In other words, in order to
correct, in accordance with an abnormal state, the torque of the electric
motor 4 (see FIG. 2) defined by the output signal IM1 (see FIG. 5),
the yaw rate feedback torque correction value calculating part 72b
calculates the yaw rate feedback torque correction value IY by
referring, relative to the yaw rate γ input, to the yaw rate
feedback map 72c defined for each vehicle speed V. The calculated yaw
rate feedback torque correction value IY is output to the adder 63
as a correction current of the torque of the electric motor 4 (see FIG.
2).

[0139]The yaw rate feedback map 72c defines the relationship between the
yaw rate γ and the yaw rate feedback torque correction value
IY, by means of the yaw rate gain GY and the vehicle speed V.
FIGS. 8A and 8B show examples of the characteristics of the yaw rate
feedback torque correction computing part 72 defining the relationship.

[0140]In FIG. 8A, in a specific range including the origin O, the yaw rate
feedback torque correction value IY linearly increases with a
specific slope as the yaw rate γ increases in a normal direction.
The yaw rate gain GY is a value that defines the slope of the
increase. It should be noted that the expression "normal direction" with
respect to the yaw rate γ herein means that, when a turning vehicle
is seen from the above, a direction of the speed of change in the
rotation angle to the turning direction of the vehicle is the same as the
direction of the change in the rotation angle, and thus the vehicle speed
is in an increasing state.

[0141]When the yaw rate γ increases in the normal direction and
exceeds a specific range, the yaw rate feedback torque correction value
IY is set so as to maintain a specific value without augmentation.
With this setting, an excessive correction to the torque of the electric
motor 4 (see FIG. 2) can be suppressed, and the assist control function
by the power steering device 110 can be surely provided.

[0142]The slope of the line in FIG. 8A becomes larger as the vehicle speed
V increases. In other words, the yaw rate feedback torque is set so as to
become larger as the vehicle speed V increases. With this setting, when
the vehicle runs at a higher speed, in order to enhance the running
stability, the yaw rate feedback torque correction value IY as a
reaction force of the torque of the electric motor 4 (see FIG. 2) is made
larger so as to make the assisting force by the power steering device 110
smaller, to thereby make the steering by the steering wheel 3 (see FIG.
1) heavier. On the other hand, when the vehicle runs at a low speed, in
order to enhance the steering feeling of the steering wheel 3 (see FIG.
1), the correction value IY is made smaller so as to make the
assisting force by the power steering device 110 larger, to thereby make
the steering by the steering wheel 3 (see FIG. 1) lighter.

[0143]In one condition of the vehicle speed V, when the yaw rate gain
GY in the case where the toe angle changers 120R and/or 120L are in
an abnormal state becomes larger than the yaw rate gain GY0 in the
case where the toe angle changers 120R, 120L are in a normal state (see
FIG. 8B), the yaw rate feedback torque is made larger as indicated with
(J) in FIG. 8B, to thereby enhance the running stability of the vehicle.
On the other hand, when the yaw rate gain GY in an abnormal state
becomes smaller than the yaw rate gain GY0 in a normal state (see
FIG. 8B), the yaw rate feedback torque is made smaller as indicated with
(K) in FIG. 8B so as to make the steering effort lighter, to thereby
enhance the operating property (handling comport).

[0144]Next, the toe angle change control diagnostic part 73 will be
described. The toe angle change control diagnostic part 73 is configured,
when receives an anomaly signal E from a self-diagnostic part 81d (which
will be described later; see FIG. 9) of the toe angle change control ECU
37 in the toe angle changer 120R (or 120L), to determine that the toe
angle changer 120R (or 120L) is in an abnormal state. In response to the
determination, the toe angle change control diagnostic part 73 forwards
the anomaly signal E to the yaw rate feedback torque correction computing
part 72, and at the same time, commands the yaw rate feedback torque
correction computing part 72 to perform corrective computation of the yaw
rate feedback torque and to output a correction signal (correction value
IY). In addition, the toe angle change control diagnostic part 73
sends the anomaly signal E to the base signal computing part 51, to
thereby allow a base signal DT to be calculated in accordance with
an abnormal state of the toe angle changers 120R (or 120L).

(Toe Angle Change Control ECU)

[0145]Next, the detailed configuration of the toe angle change control ECU
will be described with reference to FIG. 9. FIG. 9 is a block
configuration diagram showing a control function of a toe angle change
control ECU of a toe angle changer.

[0146]As shown in FIG. 9, the toe angle change control ECU 37 has a
function to drive control the electric motor 31 of the actuator 30, and
is formed of a control part 81 and an electric motor drive circuit 83.
Each toe angle change control ECU 37 is connected to the steering control
ECU 130 through a communication line, and also to the other toe angle
change control ECU 37 through a communication line.

[0147]The control part 81 includes a microcomputer with components, such
as CPU, RAM and ROM, and a peripheral circuit, and has a target current
calculating part 81a, a correction current setting part 81b, a motor
control signal generation part 81c and the self-diagnostic part (anomaly
detection unit) 81d.

[0148]The target current calculating part 81a of one of the toe angle
change controls ECU 37 is configured to calculate a target current signal
based on a signal of the target value of the toe angle of the rear wheel
2R (or 2L) input through the communication line from the target toe angle
computing part 71 of the steering control ECU 130 and to output the
result to the motor control signal generation part 81c through an adder
81e. Herein, the target current signal is a current signal required for
setting the actuator 30 so as to realize a desired operation amount of
the actuator 30 (amount of expansion/contraction of the actuator 30 that
allows the rear wheel 2R (or 2L) to have a desired toe angle). In the
present embodiment, the target current signal is also input to the
correction current setting part 81b, as a reference signal.

[0149]Based on the target value signal of the toe angle, the position
information input from the stroke sensor 38, and the target current
signal as a reference signal, the correction current setting part 81b
outputs, to the adder 81e, a correction current signal for correcting the
target current signal in accordance with a deviation from the toe angle
indicated by the target current. The adder 81e is configured to subtract
the correction current signal from the target current signal, and to
output the corrected target current signal to the electric motor.

[0150]In this manner, the target toe angle is set promptly by correcting
the target current signal from the target current calculating part 81a,
by feeding back a change in the current value required for the steering
wheel turn of the rear wheel 2R (or 2L) which change is caused by the
vehicle speed V, road conditions, motional states of the vehicle, wear
status of tire, or the like.

[0151]The motor control signal generation part 81c is configured to
receive the target current signal from the target current calculating
part 81a through the adder 81e, and to output the motor control signal to
the electric motor drive circuit 83. The motor control signal includes a
value of the current to be supplied to the electric motor 31, and a
direction of the current.

[0152]The electric motor drive circuit 83 is formed of, for example, a
bridge circuit with FET (Field Effect Transistor), and configured to
supply an electric motor voltage to the electric motor 31, based on the
motor control signal.

[0153]As shown in FIG. 9, in the steering system 100 of the vehicle of the
present embodiment, the self-diagnostic part 81d of the control part 81
is configured to receive position information of the stroke sensor 38 of
the toe angle changer 120L or 120R (to which the self-diagnostic part 81d
of interest belongs), a signal indicating a condition of the electric
motor drive circuit 83, a target current signal from the target current
calculating part 81a, and a correction current signal from the correction
current setting part 81b, and to check whether or not there is an anomaly
signal E from the other toe angle change control ECU 37 (to which the
self-diagnostic part 81d of interest does not belong). In other words,
the self-diagnostic part 81d receives and monitors signals indicating
whether or not the electric motor 31 and the electric motor drive circuit
83 in the toe angle change control ECU 37 of interest is normally
operated, and at the same time, monitors signals indicating whether or
not the electric motor 31 and the electric motor drive circuit 83 in the
other toe angle change control ECU 37 is normally operated. When the
self-diagnostic part 81d determines, based on the monitored signal, that
at least one of the electric motor 31, the stroke sensor 38, the electric
motor drive circuit 83, the target current calculating part 81a and the
correction current setting part 81b is in an abnormal state (i.e., not
operated normally), the self-diagnostic part 81d sends an anomaly signal
E to the other self-diagnostic part 81d of the other toe angle change
control ECU 37 (to which the self-diagnostic part 81d of interest does
not belong). In addition, the anomaly signal E is sent also to the toe
angle change control diagnostic part 73 of the steering control ECU 130
through the signal line. Herein, the abnormal state (failure) determined
by the self-diagnostic part 81d means, for example, a state continued for
a certain period of time, in which the toe angle indicated by the stroke
sensor 38 is apart by a specific value or more from the toe angle
corresponding to the target current signal. Such a state may occur due to
a poor turn function of the actuator 30 caused by, for example,
overheating of the electric motor 31.

[0154]In the present embodiment, the toe angle change control ECU 37
calculates the target current and is unified with the actuator 30 and
thus separately arranged from the steering control ECU 130. With this
configuration, the detected value (position information) by the stroke
sensor 38 does not have to be sent to the steering control ECU 130, and
it becomes possible to feedback-wise process the position control and
current control in the toe angle change control ECU 37. As a result, an
independent feedback loop is formed in the toe angle changer 120L (or
120R), and thus it becomes possible that settings can be made in
accordance with the individual actuator 30 in a different state from that
of the other actuator 30 (i.e. it is not necessary to make settings in
accordance with the steering control ECU 130), leading to increase in the
processing speed. In other words, the steering control ECU 130 does not
output a command including the actuation amount to the toe angle change
control ECU 37; instead, the steering control ECU 130 outputs only a
signal of the target toe angle, resulting in a minimum load on the
steering control ECU 130. Moreover, with this configuration, it becomes
easy to replace the toe angle change control ECU 37 with those having the
electric motor drive circuit 83 corresponding to the actuator 30 having
the steering effort specific to the type of the vehicle of interest.

[0155]In addition, if the electric motor 31 of the actuator 30 is
connected to the steering control ECU 130, the feedback loop becomes
significantly long, which leads to a large phase lag, resulting in poor
control accuracy. On the other hand in the present embodiment, the
control part 81 itself of the toe angle change control ECU 37 is
configured to calculate the target current, making the feedback loop
shortest, thus improving the control accuracy.

[0156]As described above, according to the present embodiment, in the
vehicle having the steering system 100 using the electric power steering
device 110, even when the toe angle changers 120R and/or 120L are in an
abnormal state and thus the vehicle becomes unstable, by performing an
assist control by the electric power steering device 110 in accordance
with the abnormal state, the vehicle can be stabilized. Specifically, in
the yaw rate feedback torque correction computing part 72, the yaw rate
gain GY in the case where the toe angle changers 120R and/or 120L
are in an abnormal state is calculated and stored, and the yaw rate
feedback torque correction value IY calculated using the yaw rate
gain GY is input to the adder 63, to thereby perform a control with
the yaw rate gain GY acting as a reaction force to the assisting
force by the electric power steering device 110.

[0157]Especially in a vehicle with a high yaw rate gain in nature, let
alone a vehicle in an abnormal state, when the steering input to the
steering wheel 3 (see FIG. 1) becomes larger, a larger yaw rate is
generated, and thus the vehicle tends to be unstable. Accordingly, by
increasing the yaw rate gain GY which defines the yaw rate feedback
torque correction value IY acting as a reaction force to the
assisting force by the electric power steering device 110, and therefore
by suppressing the assisting force and making the steering input to the
steering wheel 3 (see FIG. 1) small, it becomes possible to stabilize the
vehicle with such a property.

[0158]The embodiment of the present invention has been described above.
However, the present invention is not limited to the above embodiment,
and it is a matter of course that the above embodiment may be properly
modified without deviating from the spirit of the present invention.

[0159]For example, in the above-mentioned embodiment, the auxiliary torque
was corrected by using the correction value of feedback torque regarding
the yaw rate of the vehicle in an abnormal state. Instead, the auxiliary
torque may be corrected by using a correction value of the feedback
torque regarding parameters exhibiting vehicle properties, such as
lateral acceleration and slip angle, of the vehicle in an abnormal state.

[0160]Moreover, the determination in the yaw rate feedback torque
correction computing part 72 whether or not the toe angle changers 120R
and/or 120L are in an abnormal state may not be limited to the reception
of the anomaly signal E, and may be determined using the target value of
the toe angle generated in the target toe angle computing part 71.
Specifically, upon receiving the anomaly signal E, when the toe angle set
at the toe angle changer 120R (or 120L) does not reach the target value,
it can be determined that the toe angle changer 120R (or 120L) is in an
abnormal state.

Second Embodiment

[0161]A second embodiment of the present invention will be described with
reference to FIGS. 3, 4, and 10-15. It should be noted that components
which are the same as those illustrated in the above-mentioned embodiment
are designated with the same reference characters, and thus a duplicate
description is omitted, and that components which are equivalent,
corresponding or similar to those illustrated in the above-mentioned
embodiment, are designated with the same reference characters with a
single prime ('), and will be described in detail when necessary.

[0162]FIG. 10 is a schematic diagram of a four-wheel vehicle having a
steering system according to a second embodiment of the present
invention.

[0163]FIG. 11 is a diagram of an electric power steering device in the
steering system.

1. Configuration of Steering System

[0164]As shown in FIG. 10, a steering system 100' of the second embodiment
includes an electric power steering device 110', toe angle changers
120L', 120R', a steering controller (hereinbelow, referred to as
"steering control ECU") 130', and various sensors including a front wheel
turning angle sensor SS and a vehicle speed sensor SV. Other
components are substantially the same as in the steering system according
to the first embodiment shown in FIG. 1, and when different, the
component will be described.

2. Configuration of Electric Power Steering Device

[0165]The electric power steering device 110' of the present embodiment is
substantially the same as the electric power steering device of the first
embodiment shown in FIG. 2, except that, as shown in FIG. 11, the
electric power steering device 110' includes the steering control ECU
130' and the steering wheel turn mechanism includes the front wheel
turning angle sensor Ss for detecting a direction of the front wheels 1L,
1R, based on an amount of movement of the rack shaft 8 in a vehicle width
direction.

[0166]The electric motor steering control ECU 130' of the steering system
100' has an electric power steering control part 130a' (which will be
described later; see FIG. 13) as a functional part of the electric power
steering device 110', which controls the driving of the electric motor 4.

3. Configuration of Toe Angle Changer

[0167]Next, a configuration of the toe angle changer will be described
with reference to FIG. 12.

[0168]FIG. 12 is a schematic cross sectional view showing a structure of
an actuator of a toe angle changer.

[0169]It should be noted that the toe angle changer 120L' in the present
embodiment is different from the toe angle changer 120L according to the
first embodiment shown in FIG. 3, in that the actuator 30, the toe angle
change control ECU 37, and the steering control ECU 130 in the first
embodiment are replaced with an actuator 30', a toe angle change control
ECU 37' and the steering control ECU 130', respectively. The other
components are the same as those in the first embodiment and thus a
duplicate description is omitted. The configurational arrangement of all
components including the corresponding components is the same as that in
the first embodiment (see FIG. 3), and thus the duplicate drawing and the
duplicate description are omitted.

[0170]The actuator 30' in the present embodiment is the same as the
actuator 30 in the first embodiment shown in FIG. 4, except that the
actuator 30' has an electric motor 31'. The electric motor 31' is the
same as the electric motor 31 in the first embodiment shown in FIG. 4,
except that the electric motor 31' further has a temperature sensor 31a.
The temperature sensor 31a is configured to detect (measure) a winding
temperature of a coil of the electric motor 31', and to input a detected
temperature signal to a self-diagnostic part 81d' (see FIG. 15), which
will be described later, of the toe angle change control ECU 37'.

[0171]Herein, the self-diagnostic part 81d' and the temperature sensor 31a
constitute an anomaly detection unit of the present invention.

[0172]It should be noted that the toe angle change control ECU 37' is
fixed to the case body 34 of the actuator 30', and connected to the
stroke sensor 38 and the temperature sensor 31a through connectors or the
like.

4. Functional Configuration of Steering Control ECU

[0173]Next, functions of the steering control ECU will be described with
reference to FIG. 13.

[0174]FIG. 13 is a schematic diagram of a control function of a steering
control ECU and toe angle changers in the steering system according to
the second embodiment.

[0175]As shown in FIG. 13, the steering control ECU 130' includes: the
electric power steering control part 130a' configured to control the
electric power steering device 110' (see FIGS. 10 and 11); and a rear
wheel toe angle control part 130b' configured to compute target toe
angles of the rear wheels 2L, 2R.

(Electric Power Steering Control Part)

[0176]With respect to the electric power steering control part 130a', only
the portions different from those of the first embodiment, and
configurations associated therewith, will be described with reference to
FIG. 13.

[0177]The output signal IM1 of the adder 62 is a target signal for
the Q-axis current which defines the torque of the electric motor 4, and
input to an anomaly auxiliary torque restriction part 163, which
restricts the auxiliary torque when the toe angle changer(s) is in an
abnormal state (unlike the first embodiment, the electric power steering
control part of the present embodiment does not have the adder 63).

[0178]The anomaly auxiliary torque restriction part 163 is configured to
reduce the signal IM1 from the adder 62 at the moment a correction
command signal is input from a toe angle change control diagnostic part
73' (which will be described later), by a specific correction amount
ΔIM as shown in FIG. 14 and to output the output signal IM2 to
the adder 64.

[0179]The magnitude of the correction amount ΔIM is set to be a
specific percent, such as 80%, of the signal IM1. The current
changes between positive and negative when the steering wheel is turned
left and right, respectively, but regardless of the sign (positive or
negative) of the signal IM1, by diminishing the signal IM1 to
80% of the input value, the auxiliary torque can be reduced in such a
manner that the driver can sense the reduction.

[0180]When the correction command signal is not input from the toe angle
change control diagnostic part 73', the anomaly auxiliary torque
restriction part 163 outputs the signal IM1 as-is as an output
signal IM2 to the adder 64.

(Rear Wheel Toe Angle Control Part)

[0181]Next, the rear wheel toe angle control part 130b' will be described
with reference to FIG. 13. As shown in FIG. 13, the rear wheel toe angle
control part 130b' includes a target toe angle computing part 71' and the
toe angle change control diagnostic part 73'.

[0182]The front wheel turning angle sensor Ss is configured to detect
(measure) a turning angle δ of the front wheels 1L, 1R and to input
the result to the target toe angle computing part 71'.

[0183]The target toe angle computing part 71' is configured to generate
target toe angles αTL, αTR for the rear wheels 2L,
2R, respectively, based on the vehicle speed signal VS, a turning angle
δ, and a turning angular velocity δ' which is easily obtained
by temporal differentiation of the turning angle δ, and to input
the target toe angles αTL, αTR to the respective
toe angle change control ECUs 37', 37' configured to control respective
toe angle changes of the left rear wheel 2L and the right rear wheel 2R
(see FIG. 15). The target toe angles αTL, αTR are
generated from a toe angle table 71a', with reference to the turning
angle δ, the turning angular velocity δ' and the vehicle
speed V, which table had been prepared for each of the left rear wheel 2L
and the right rear wheel 2R in advance.

[0184]For example, the target toe angles αTL, δTR
are defined by the following formulae (6) and (7):

TL=KL(V,δ',δ)δ (6)

TR=KR(V,δ',δ)δ (7)

[0185]where each of KL(V,δ',δ),
KR(V,δ',δ) represents a front-rear wheel steering ratio
which depends on the vehicle speed V, the turning angle δ and the
turning angular velocity δ' of the turning angle. When the vehicle
speed V is in a specific low-speed range, each of the target toe angles
αTL, αTR of the rear wheel is generated in such a
manner that the rear wheels 2L, 2R are in antiphase relative to the front
wheels, in accordance with the turning angle δ, to allow the
vehicle to turn in a small radius.

[0186]In the high-speed range over the above-mentioned specific low-speed
range, when an absolute value of the turning angular velocity δ' is
a specific value or less, and at the same time, the turning angle δ
is within a specific range (including right and left), the target toe
angles αTL, αTR of the rear wheels 2L, 2R are set
as the same phase relative to the front wheels, in accordance with the
turning angle δ. In other words, the target toe angles
αTL, αTR of the rear wheels 2L, 2R are set so as to
make the slip angle β small during lane change.

[0187]However, in the high-speed range over the above-mentioned specific
low-speed range, when the absolute value of the turning angular velocity
δ' exceeds a specific value, or when the turning angle δ is
too large to fall outside the specific range (including right and left),
the target toe angles αTL, αTR of the rear wheels
are set to antiphase relative to the front wheels, in accordance with the
turning angle δ.

[0188]It should be noted that, from the viewpoint of the stability in a
turn, the target toe angles αTL, αTR generated in
the target toe angle computing part 71' do not necessarily follow
Ackerman-Jeantaud geometry. Further, when the turning angle δ is
0°, each of the target toe angles αTL, αTR
may be, for example, 2°, with the wheels toed in.

[0189]Next, the toe angle change control diagnostic part 73' will be
described. The toe angle change control diagnostic part 73' is configured
to receive an anomaly detection signal (which will be described later)
from a self-diagnostic part 81d' (which will be described later; see FIG.
15) of the toe angle change control ECU 37' of the toe angle changers
120L' and/or 120R', and to determine whether or not the specific actual
toe angles αSL, αSR (which will be described later)
received together with the anomaly detection signals indicate that at
least one of the rear wheels 2L, 2R is in a toe-out state (i.e., not both
of the specific actual toe angles αSL, αSR are
selected from zero and those indicating a toe-in state), and at the same
time, whether or not the vehicle speed is a specific value Vlow or
lower. When these conditions are satisfied, the toe angle change control
diagnostic part 73' outputs the correction command signal to the anomaly
auxiliary torque restriction part 163 so that the signal IM1 input
to the anomaly auxiliary torque restriction part 163 is reduced by a
specific correction amount ΔIM as described above, which is then
output as the signal IM2. When the above-mentioned conditions are
not satisfied, for example, when the both of the rear wheels are locked
in the toe-in state, or when one of the rear wheels is locked in the
toe-in state and the other has the specific actual toe angle of zero
(αSL=0 or αSR=0), or when the rear wheels are
locked with the specific actual toe angles
αSL=αSR=0, or when both of the toe angle changers
120L', 120R' are in a normal state, the correction command signal is not
output to the anomaly auxiliary torque restriction part 163.

(Toe Angle Change Control ECU)

[0190]Next, the detailed configuration of the toe angle change control ECU
will be described with reference to FIG. 15. FIG. 15 is a block
configuration diagram showing a control function of a toe angle change
control ECU of a toe angle changer.

[0191]As shown in FIG. 15, the toe angle change control ECU 37' has a
function of drive control of the actuator 30', and is formed of a control
part 81' and the electric motor drive circuit 83. Each toe angle change
control ECU 37' is connected to the steering control ECU 130' through a
communication line, and also to the other toe angle change control ECU
37' through a communication line.

[0192]The control part 81' includes a microcomputer with components, such
as CPU, RAM, ROM, and a peripheral circuit, and has a target current
calculating part 81a, a motor control signal generation part 81c and the
self-diagnostic part (anomaly detection unit) 81d'.

[0193]The target current calculating part 81a of one toe angle change
control ECU 37' (on a right rear wheel 2R side) is configured to
calculate a target current signal based on the target toe angle
αTR of the rear wheel 2R input through the communication line
from the steering control ECU 130' and on the present toe angle
αR of the rear wheel 2R obtained from the stroke sensor 38,
and to output the result to the motor control signal generation part 81c.

[0194]The target current calculating part 81a of the other toe angle
change control ECU 37' (on a left rear wheel 2L side) is configured to
calculate a target current signal based on the target toe angle
αTL of the rear wheel 2L input through the communication line
from the steering control ECU 130' and on the present toe angle
αL of the rear wheel 2L obtained from the stroke sensor 38,
and to output the result to the motor control signal generation part 81c.

[0195]Herein, the target current signal is a current signal required for
setting the actuator 30' so as to realize a desired operation amount of
the actuator 30' (amount of expansion/contraction of the actuator 30'
that allows the rear wheel 2R (or 2L) to have a desired toe angle
αTR (or αTL)) at a desired speed.

[0196]In this manner, the target toe angle αTR (or
αTL) is set promptly in the target current calculating part
81a, by feeding back the present toe angle αTR (or
αTL) and the target toe angle αTR (or
αTL) and correcting the target current signal, and by feeding
back a change in the current value required for the steering wheel turn
of the rear wheel 2R (or 2L) which change is caused by the vehicle speed
V, road conditions, motional states of the vehicle, wear status of tire,
or the like.

[0197]It should be noted that the response may be enhanced in the target
current calculating part 81a, by adding feedforward control.

[0198]The motor control signal generation part 81c is configured to
receive the target current signal from the target current calculating
part 81a, and to output the motor control signal to the electric motor
drive circuit 83. The motor control signal includes a value of the
current to be supplied to the electric motor 31, and a direction of the
current.

[0199]The electric motor drive circuit 83 is formed of, for example, a
bridge circuit with FET (Field Effect Transistor), and configured to
supply an electric motor current to the electric motor 31, based on the
motor control signal.

[0200]As shown in FIG. 15, the self-diagnostic part 81d' is configured to
determine whether or not an abnormal state is detected, based on a
position signal of the stroke sensor 38 of the toe angle changer 120L' or
the toe angle changer 120R' (to which the self-diagnostic part 81d' of
interest belongs), a detection signal from a Hall element of the electric
motor drive circuit 83, a temperature signal from the temperature sensor
31a, and a state monitoring of the target current calculating part 81a.

[0201]For example, the self-diagnostic part 81d' determines that a winding
temperature of the electric motor 31' is in an abnormal state (high
winding temperature mode) when the signal from the temperature sensor 31a
exceeds a specific value, and inputs a specific actual toe angle
αSL (or αSR), such as 0°, to the target
current calculating part 81a. Herein, the actual toe angles
αSL and αSR are actual toe angles regarding the
left rear wheel 2L and the right rear wheel 2R, respectively, which are
realized by inputting an actual toe angle when an abnormal state is
detected.

[0202]The self-diagnostic part 81d' is configured to monitor the target
current value from the target current calculating part 81a and the
detection signal of actual current from a Hall element of the electric
motor drive circuit 83, and to determine whether or not the actuator 30'
is locked, based on the position signal from the stroke sensor 38: when
it is determined that the actuator 30' is locked (lock mode), the
self-diagnostic part 81d' commands the electric motor drive circuit 83 to
stop the power supply to the electric motor 31', and inputs the present
toe angle αL (or αR) as the actual toe angle
αSL (or αSR) to the target current calculating part
81a, and then sends an anomaly detection signal and a signal of the
above-mentioned mode made in response to the anomaly detection, to the
self-diagnostic part 81d' of the other toe angle change control ECU 37'.

[0203]It should be noted that, for an anomaly detection unit, a watch dog
circuit may be provided as a peripheral circuit in addition to the
self-diagnostic part 81d', to monitor the control part 81'. In this case,
when an abnormal state of the control part 81' is detected, the electric
motor drive circuit 83 may be commanded to stop a power supply to the
electric motor 31', and then an anomaly detection signal may be output to
the self-diagnostic part 81d' of the other toe angle change control ECU
37'.

[0204]In addition, the self-diagnostic part 81d' of the toe angle changer
120L' (or 120R') is configured to check whether or not there is an
anomaly detection signal from the self-diagnostic part 81d' of the toe
angle change control ECU 37' of the other toe angle changer 120R' (or
120L'). When the anomaly detection signal is received, the actual toe
angle αSL (or αSR) is input to the target current
calculating part 81a, based on a signal of the above-mentioned process
mode.

[0205]In other words, the self-diagnostic part 81d' monitors a signal
indicating whether or not the toe angle changer 120L' (or 120R')
corresponding to the toe angle change control ECU 37' of interest is
normally operated, and at the same time, monitors a signal indicating
whether or not the toe angle changer 120R' (or 120L') corresponding to
the other toe angle change control ECU 37' is normally operated. When one
of the toe angle changers 120' is found to be in an abnormal state, both
of the toe angle change control ECUs 37', 37' perform a process in the
same specific mode.

[0206]Then, each self-diagnostic part 81d' sends an anomaly detection
signal, a signal of the above-mentioned mode made in response to the
anomaly detection, and a specific actual toe angle αSL (or
αSR) to the toe angle change control diagnostic part 73'.

[0207]Next, a control in the toe angle change control diagnostic part 73'
will be described with reference to FIG. 16. FIG. 16 is a flow chart
showing a control in the toe angle change control diagnostic part. This
control process is performed in a predetermined cycle.

[0208]In a step S11, a vehicle speed V, an anomaly detection signal and
specific actual toe angles αSL, αSR from the
self-diagnostic part 81d' of the right and left toe angle change control
ECUs 37' are constantly read.

[0209]In a step S12, it is checked whether or not an anomaly detection
signal is received (anomaly detection ?). When an abnormal state is
detected (Yes), the procedure advances to a step S13, and when the
abnormal state is not detected (No), a procedure in one cycle is
terminated.

[0210]In the step S13, the specific actual toe angle αSL,
αSR read in the step S11 is examined to determine whether or
not both of the left rear wheel 2L and the right rear wheel 2R are in a
toe-in state or with the specific actual toe angle of zero. When both of
the rear wheels 2L, 2R are either in the toe-in state or with the
specific actual toe angle of zero (i.e., when both of the rear wheels 2L,
2R are in the toe-in state, or when both of the specific actual toe
angles αSL, αSR are zero, or when one of the rear
wheels 2L, 2R is in the toe-in state and the other has the specific
actual toe angle of zero) (Yes), a procedure in one cycle is terminated.
When the above mentioned condition is not satisfied (i.e., at least one
of the rear wheels 2L, 2R is in a toe-out state) (No), the procedure
advances to a step S14 where it is determined whether or not the vehicle
speed V is a specific vehicle speed Vlow or lower. When the vehicle
speed V is larger than the specific vehicle speed Vlow (No), a
procedure in one cycle is terminated. When the vehicle speed V is the
specific vehicle speed Vlow or lower (Yes), the correction command
signal is output to the anomaly auxiliary torque restriction part 163. In
other words, the auxiliary torque is reduced, and then a series of the
control is terminated.

[0211]Subsequently, the anomaly auxiliary torque restriction part 163
gently reduces the input signal IM1 to a specific percentage of the
auxiliary torque with, for example, a specific delay as shown in FIG. 14,
and then output the signal IM2, which is set to be a specific
percent (20% in this case) of the signal IM1, to the adder 64.

[0212]It should be noted that the vehicle speed Vlow is a
sufficiently low speed (e.g. the lowest running speed of 10 km/hr) which
does not affect the vehicle run, even when an awkward feeling is given to
the driver as a result of a sudden change in the steering effort, in
response to the sudden change in the auxiliary torque, such as 80%
reduction.

[0213]As described above, according to the present embodiment, when an
anomaly detection signal regarding the toe angle changer is received from
the anomaly detection unit, the target value can be reduced so that the
responsive feeling from the steering torque becomes large. In addition,
when an anomaly detection signal is received from the anomaly detection
unit, and at the same time, when the vehicle speed is a specific value or
less, the target value of the auxiliary torque is reduced. As a result,
even though the vehicle runs at a relatively high speed, it is possible
to prevent a sudden increase in the responsive feeling from the steering
torque. In addition, the calculated target value of the auxiliary torque
is reduced only when the steering controller receives an anomaly
detection signal from the anomaly detection unit, the vehicle speed is a
specific value or lower, and at least one of the left and right toe
angles of the rear wheels is locked in a toe-out state.

[0214]Specifically, when the toe angle change control diagnostic part 73'
receives an anomaly detection signal from the toe angle change control
ECU 37', and when both of the rear wheels 2L, 2R are not in a toe-in
state or when the actual toe angles αSL=αSR=0 is
not satisfied or when it is not the case that the one of the rear wheels
is in a toe-in state and the actual toe angle of the other rear wheel is
zero, the steering control ECU 130' makes the adder 64 output in such a
manner that the auxiliary torque is reduced, after confirming that the
vehicle speed is Vlow or lower. With this configuration, the
auxiliary torque becomes smaller at a sufficiently low speed, and the
responsive feeling from the steering torque given to the driver becomes
large, making it easier for the driver to sense an abnormal state of the
steering function.

[0215]Therefore, an awkward feeling, which may otherwise be given to the
driver as a result of a sudden reduction in the auxiliary torque during
running, can be prevented.

[0216]Since the gain is set in such a manner that the auxiliary torque is
reduced in accordance with the increase in the speed, if the vehicle runs
at a vehicle speed exceeding the vehicle speed Vlow, an anomaly in
the toe angle changers 120L' and/or 120R' is easily sensed by the driver.
Therefore, the driver can easily reduce the vehicle speed in response to
the discomfort feeling in steerability, and when the vehicle speed
reaches Vlow or lower, the auxiliary torque is reduced, to thereby
surely inform the driver of anomaly in the toe angle changers 120L'
and/or 120R'.

[0217]In this case, the auxiliary torque is reduced with a time delay, and
thus it is possible to prevent the driver from being surprised at a rapid
decrease in the auxiliary torque.

[0218]It should be noted that, the reason for not reducing the auxiliary
torque when the signal from the toe angle change control ECU 37'
indicates that the rear wheels 2L, 2R are in a toe-in state or
αSL=αSR=0 is satisfied is that, when the rear
wheels 2L, 2R are locked with αSL=αSR=0,
steerability and turnability are the same as those of a conventional
vehicle with the steering of only front wheels 1L, 1R, and when the
vehicle is in the toe-in state, vehicle behavior rather shows properties
on the safe side. Therefore, it would suffice to warn the driver by
turning on an alarm lamp provided on a console, which informs an anomaly
in the toe angle changers 120' and/or 120R'.

[0219]When the self-diagnostic part 81d' of one of the toe angle change
control ECUs 37', 37' detects an abnormal state, the self-diagnostic part
81d' sends an anomaly detection signal to the other toe angle change
control ECU 37', and both of the toe angle changers 120L', 120R' are
controlled so that both of the toe angles are fixed. Therefore, it is
prevented that a change of only one of the toe angles between the rear
wheels 2L, 2R remains controlled, and thus a driving performance is
maintained stable even when the toe angle changers 120L', 120R' are in an
abnormal state.

(Modified Versions of Second Embodiment)

[0220]The present invention is not limited to the second embodiment, and
it is possible to make, for example, the following various modifications.

(1) In the electric power steering control part 130a' according to the
second embodiment, the current in the electric motor 4 is controlled by
setting the target current. Instead, a target voltage may be set as a
voltage to be applied to the electric motor 4. Alternatively, a target
torque may be set as a torque to be output by the electric motor 4, to
thereby control the current in the electric motor 4. Such a target
voltage and a target torque are included in the target signal.(2) In the
second embodiment, the toe angle change control diagnostic part 73'
outputs the correction command signal to the anomaly auxiliary torque
restriction part 163, so as to make the anomaly auxiliary torque
restriction part 163 reduce the current target value IM1. However,
the present invention is not limited to this embodiment.

[0221]As shown in a steering control ECU 130'A in FIG. 17, a relay switch
60 may be disposed downstream from the PWM conversion part 57 and an
auxiliary torque may be cut off by turning off the relay switch 60, in
response to the command from the toe angle change control diagnostic part
73'. With this configuration, when the anomaly detection unit detects an
abnormal state of the toe angle changer, the amount of the auxiliary
torque becomes 0, leading to a large responsive feeling of the steering
torque. Moreover, the steering control ECU 130'A may be set in such a
manner that the auxiliary torque becomes 0 when the vehicle speed is a
specific value or lower. With this configuration, sudden increase in the
responsive feeling of the steering torque can be prevented when the
vehicle runs at a relatively high speed, and thus an awkward feeling,
which may otherwise be given to the driver, can be prevented.
Furthermore, the steering control ECU 130'A may be set in such a manner
that the auxiliary torque becomes 0 only when at least one of the toe
angles of the left and right rear wheels is fixed in a toe-out state
(i.e., only when the toe angles are fixed such that a stable turnability
is given, with none of the left and right rear wheels being toed out).
With this configuration, there can be provided a rational steering system
in which an unnecessary reduction (including zeroing) of the auxiliary
torque is prevented.

(3) In the second embodiment, as shown in FIG. 6A, the base signal
computing part 51 generates the base signal DT to be used as a
standard reference for a target signal IM1 for the output torque
TM*, based on the torque signal T and the vehicle speed signal VS as
a parameter, but the present invention is not limited to this embodiment.
The present invention can be applied to an electric power steering device
in which: an operation angle signal from an operation angle sensor
provided in the steering wheel 3 and a yaw rate signal from a yaw rate
sensor provided in the vehicle body are input to the steering control ECU
130'; the reference yaw rate, which is determined in advance based on the
vehicle speed signal VS and the operation angle, is calculated; and
feedback control of the reaction force to the steering wheel 3 is
performed based on a difference between the reference yaw rate and the
actual yaw rate.

Third Embodiment

[0222]A third embodiment of the present invention will be described with
reference to FIGS. 3, 6A-6C, 12, 15 and 18-24. It should be noted that
components which are the same as those illustrated in the above-mentioned
embodiments are designated with the same reference characters, and thus a
duplicate description is omitted, and that components which are
equivalent, corresponding or similar to those illustrated in the
above-mentioned embodiments, are designated with the same reference
characters with a double prime (''), and will be described in detail when
necessary.

[0223]FIG. 18 is a schematic diagram of a four-wheel vehicle having a
steering system according to a third embodiment of the present invention.
FIG. 19 is a diagram of an electric power steering device in the steering
system.

1. Configuration of Steering System

[0224]As shown in FIG. 18, a steering system 100'' of the third embodiment
includes an electric power steering device 110'', toe angle changers
120L'', 120R'', a steering controller (hereinbelow, referred to as
"steering control ECU") 130'', and various sensors, including a vehicle
speed sensor SV. Other components are substantially the same as in
the steering system according to the first embodiment shown in FIG. 1,
and when different, the component will be described.

2. Configuration of Electric Power Steering Device

[0225]The electric power steering device 110'' of the present embodiment
is substantially the same as the electric power steering device of the
first embodiment shown in FIG. 2, except that, as shown in FIG. 19, the
electric power steering device 110'' includes the steering control ECUs
130''.

[0226]The electric motor steering control ECU 130'' of the steering system
100'' has an electric power steering control part 130a'' (which will be
described later; see FIG. 20) as a functional part of the electric power
steering device 110'', which controls the driving of the electric motor
4.

3. Configuration of Toe Angle Changer

[0227]Next, a configuration of the toe angle changer will be described.

[0228]It should be noted that the toe angle changer 120L'' in the present
embodiment is different from the toe angle changer 120L according to the
first embodiment shown in FIG. 3, in that the actuator 30, the toe angle
change control ECU 37, and the steering control ECU 130 in the first
embodiment are replaced with an actuator 30'', a toe angle change control
ECU 37'' and the steering control ECU 130'', respectively. The other
components are the same as those in the first embodiment, and thus a
duplicate description is omitted. The configurational arrangement of all
components including the corresponding components is the same as that in
the first embodiment (see FIG. 3), and thus the duplicate drawing and the
duplicate description are omitted.

[0229]The actuator 30'' in the present embodiment is the same as the
actuator 30' in the second embodiment shown in FIG. 12, and thus the
duplicate drawing and the duplicate description are omitted.

4. Functional Configuration of Steering Control ECU

[0230]Next, functions of the steering control ECU will be described with
reference to FIGS. 20 and 6A-6C.

[0231]FIG. 20 is a schematic diagram of a control function of a steering
control ECU and toe angle changers in the steering system according to
the third embodiment. FIG. 6A is a graph showing a function of base
signal stored in a base table, FIG. 6B is a graph showing a
characteristic function of a damper table, and FIG. 6C is a graph showing
relationships among a damper table N, a damper table A and a damper table
B.

[0232]As shown in FIG. 20, the steering control ECU 130'' includes: the
electric power steering control part 130a'' configured to control the
electric power steering device 110''; and a rear wheel toe angle control
part 130b'' configured to compute target toe angles of the rear wheels
2L, 2R.

(Electric Power Steering Control Part)

[0233]With respect to the electric power steering control part 130a'',
only the portions different from those of the first embodiment, and
configurations associated therewith, will be described with reference to
FIGS. 20, 6A and 6B (and FIG. 2 where appropriate).

[0234]Referring to FIG. 20, a base signal computing part 51'' in the
present embodiment further stores a backup table 51b, in addition to the
base signal computing part 51 with the base table 51a of the first
embodiment, and the base signal computing part 51'' generates the base
signal (target value) DT to be used as a standard reference for the
target signal IM1 of the output torque TM*, based on the torque
signal T and the vehicle speed signal VS using the backup table 51b, in
response to a command from a toe angle change control diagnostic part
73'' (which will be described below), when the toe angle changers 120L'',
120R'' are in an abnormal state.

[0235]The backup table 51b has a function of the torque signal T and the
vehicle speed signal VS as also shown in FIG. 6A, but the values of the
gains (G1, G2) are smaller by notable amounts than those in the case of
the base table 51a, for the same vehicle speed. With this setting, the
auxiliary torque becomes smaller, making it easier for the driver to
sense an abnormal state.

[0236]In the present embodiment, a damper compensation signal computing
part 52'' is introduced for compensating a viscosity in the steering
unit, and for providing a steering damper function for compensating
convergence of steering wheel position when convergence decreases during
high-speed driving, by reading a damper table 52a'' with reference to the
angular velocity signal ω. In the present embodiment, in addition
to a normal damper table N 52a1 (first table), which is to be referred to
when the toe angle changer 120'' is in a normal state, the damper table
52a'' further stores a damper table A 52a2 (second table) and a damper
table B 52a3 (third table) (which will be described below), which are to
be referred to when the toe angle changer 120'' is in an abnormal state.
The damper compensation signal computing part 52'' is also configured to
receive information of anomaly of the toe angle changer 120'' from the
toe angle change control diagnostic part 73'', and to switch the tables
in the damper table 52a'' to be referred to by the steering control ECU
130'', based on the input information (see FIG. 6B for the property of
the damper table, and see the first embodiment for the description
thereof).

[0237]FIG. 6C is a graph showing relationships between angular velocity
ω and damping compensation value I stored in the damper table N,
the damper table A and the damper table B. The damper table A 52a2 shown
in FIG. 20 is a table which is referred to when an anomaly, as a toe-in
failure state, is detected in the toe angle changer 120, and is used for
setting a smaller compensation value I than the value set by the damper
table N 52a1 (see FIG. 20). Therefore, as indicated with a broken line in
FIG. 6C, a proportion gain (increasing rate relative to the increase in
the angular velocity ω) is set smaller as compared with the damper
table N 52a1. On the other hand, the damper table B 52a3 shown in FIG. 20
is a table which is referred to when an anomaly, as a toe-out failure
state, is detected in the toe angle changer 120, and is used for setting
a larger compensation value I than the value set by the damper table N
52a1. Therefore, as indicated with a dashed-dotted line in FIG. 6C, a
proportion gain (increasing rate relative to the increase in the angular
velocity ω) is set larger as compared with the damper table N 52a1.
It should be noted that the expression "toe-in failure state" herein
means that a state in which the rear wheels 2 are locked in a toe-in
state, while the expression "toe-out failure state" herein means that a
state in which the rear wheels 2 are locked in a toe-out state.

[0238]The adder 64 is configured to subtract the Q-axis current IQ from
the output signal IM1 from the adder 62, and to generate a deviation
signal IE (unlike the first embodiment, the electric power steering
control part of the present embodiment does not have the adder 63).

(Rear Wheel Toe Angle Control Part)

[0239]Next, with respect to the rear wheel toe angle control part 130b'',
only the portions different from those of the second embodiment, and
configurations associated therewith, will be described with reference to
FIG. 20. As shown in FIG. 20, the rear wheel toe angle control part
130b'' includes a front wheel turning angle computing part 68, a target
toe angle computing part 71'' and the toe angle change control diagnostic
part 73''.

[0240]The front wheel turning angle computing part 68 is configured to
calculate a turning angle δ of the front wheels 1L, 1R based on the
angular signal θ of the electric motor 4 output from the resolver
25, and to input the result to the target toe angle computing part 71''
and the toe angle change control diagnostic part 73''.

[0241]The target toe angle computing part 71'' is similar to that of the
target toe angle computing part 71' in the second embodiment, and the
duplicate description is omitted.

[0242]The toe angle change control diagnostic part 73'' is configured to
receive an anomaly detection signal and a toe angle αL (or
αR) of the rear wheel 2 from a self-diagnostic part 81d''
(which will be described below) of the toe angle change control ECU 37''
of the toe angle changer 120L'' (or 120R''). The toe angle change control
diagnostic part 73'' is also configured to receive a turning angle
δ of the front wheel 1 output from the front wheel turning angle
computing part 68 and to compare the turning angle δ with the toe
angle αL (or αR) of the rear wheel 2, in terms of
direction and magnitude.

(Toe Angle Change Control ECU)

[0243]The configuration of the toe angle change control ECU 37'' in the
present embodiment is substantially the same as that of the toe angle
change control ECU 37' in the second embodiment shown in FIG. 15, and
thus the duplicate drawing and the duplicate description are omitted.
Only the self-diagnostic part 81d'' having a different function from that
in the second embodiment, and configurations associated therewith, will
be described in detail.

[0244]The self-diagnostic part (anomaly detection unit) 81d'' is
configured to determine whether or not an abnormal state is detected,
based on a position signal of the stroke sensor 38 of the toe angle
changer 120L'' or the toe angle changer 120R'' (to which the
self-diagnostic part 81d'' of interest belongs, see FIG. 18), a detection
signal from a Hall element of the electric motor drive circuit 83, a
temperature signal from the temperature sensor 31a, and a state
monitoring of the target current calculating part 81a.

[0245]For example, the self-diagnostic part 81d'' determines that a
winding temperature of the electric motor 31' is in an abnormal state
when the signal from the temperature sensor 31a exceeds a specific value,
and inputs a specific target toe angle αTL (or
αTR), such as 0°, to the target current calculating
part 81a. Herein, the target toe angles αTL and αTR
are target toe angles regarding the left rear wheel 2L and the right rear
wheel 2R, respectively, when an abnormal state is detected.

[0246]The self-diagnostic part 81d'' is configured to monitor the
detection signals from the target current calculating part 81a and a Hall
element of the electric motor drive circuit 83, and to determine whether
or not the actuator 30'' is locked, based on the position signal from the
stroke sensor 38: when it is determined that the actuator 30'' is locked,
the self-diagnostic part 81d'' commands the electric motor drive circuit
83 to stop the power supply to the electric motor 31', and inputs the
present toe angle αL (or αR) as the target toe
angle αTL (or αTR) to the target current
calculating part 81a, and then sends an anomaly detection signal and a
signal of the above-mentioned mode made in response to the anomaly
detection, to the self-diagnostic part 81d'' of the other toe angle
change control ECU 37''.

[0247]It should be noted that, for an anomaly detection unit, a watch dog
circuit may be provided as a peripheral circuit in addition to the
self-diagnostic part 81d'', to monitor the control part 81''. In this
case, when an abnormal state of the control part 81'' is detected, the
electric motor drive circuit 83 may be commanded to stop a power supply
to the electric motor 31', and then an anomaly detection signal may be
output to the self-diagnostic part 81d'' of the other toe angle change
control ECU 37''.

[0248]In addition, the self-diagnostic part 81d'' of the toe angle changer
120L'' (or 120R'') is configured to check whether or not there is an
anomaly detection signal from the self-diagnostic part 81d'' of the toe
angle change control ECU 37'' of the other toe angle changer 120R'' (or
120L''). When the anomaly detection signal is received, the target toe
angle αTL (or αTR) is input to the target current
calculating part 81a, based on a signal of the above-mentioned process
mode.

[0249]In other words, the self-diagnostic part 81d'' monitors a signal
indicating whether or not the toe angle changer 120L'' (or 120R'')
corresponding to the toe angle change control ECU 37'' of interest is
normally operated, and at the same time, monitors a signal indicating
whether or not the toe angle changer 120R'' (or 120L'') corresponding to
the other toe angle change control ECU 37'' is normally operated. When
one of the toe angle changers 120'' is found to be in an abnormal state,
both of the toe angle change control ECUs 37'', 37'' perform a process in
the same specific mode.

[0250]Then, each self-diagnostic part 81d'' sends an anomaly detection
signal to the toe angle change control diagnostic part 73''.

[0251]In a vehicle having the steering system 100'' (see FIG. 18)
including the components as described above, the damping control is
performed in accordance with the steering wheel turn of the front wheel 1
by the operation of the steering wheel 3 (see FIG. 18) by the driver, as
will be described below (see the drawings where appropriate).

[0252]FIGS. 21A and 21B show results obtained from a vehicle run on a
slalom road: FIG. 21A is a graph showing relationships between turning
angle and yaw rate for different toe angles of rear wheels, and FIG. 21B
is a graph showing relationships between turning angle and steering
torque for different toe angles of rear wheels.

[0253]As shown in FIG. 21A, when the toe angles αL,
αR of the rear wheels 2L, 2R are set as toe-in, the yaw rate
relative to the turning angle δ of the front wheel 1 becomes
smaller as compared with the case where the toe angles αL,
αR of the rear wheels 2L, 2R are 0°. In other words,
the yaw rate gain Gγ shown as a magnitude of the yaw rate relative
to the turning angle δ is lower.

[0254]Since the yaw rate gain Gγ is a magnitude of the yaw rate of
the vehicle corresponding to the turning angle δ as described
above, when the yaw rate gain Gγ is lower, the follow-up property
of the yaw rate of the vehicle relative to the operation of the steering
wheel 3 by the driver becomes lower.

[0255]Herein, when the damping control corresponding only to the turning
angle δ of the front wheels 1 is performed, the damping control is
performed under the assumption that the toe angles αL,
αR of the rear wheels 2L, 2R are 0°, and therefore, a
phase in the case of damping control advances relative to the case of a
lower yaw rate gain Gγ obtained when the toe angles αL,
αR of the rear wheels 2L, 2R are set as toe-in.

[0256]FIG. 21B is a graph showing relationships between turning angle and
steering torque, when a vehicle runs on a slalom road. As the turning
angle δ changes, the steering torque TS changes along a locus
in the graph in a clockwise direction as indicated with arrows. As shown
in FIG. 21B, when the rear wheels 2 are toed in, the locus thereof
(indicated with a broken line in the graph) locates outside a locus in
the case where the toe angles αL, αR of the rear
wheels 2L, 2R are 0° (indicated with a solid line in the graph).

[0257]In FIG. 21B, it is assumed that the turning angle δ is
oriented in a positive direction, and that a point A on the broken line
and a point B on the solid line are the same in terms of the steering
torque TS. With respect to the turning angle δ, the point B is
larger than the point A. To put it another way, the steering torque
TS corresponding to the arbitrary turning angle δ (point B) in
the case where the toe angle of the rear wheel 2 is 0° will have a
corresponding smaller turning angle δ (point A) than that in the
case where the toe angle is 0°, if the rear wheels 2 are made toed
in. Since both of the point A and the point B are on their respective
locus for the positive direction, i.e., the direction of steering that
makes the turning angle δ larger, when the rear wheels 2 are made
toed in, the follow-up of the steering torque TS in accordance with
the change in the turning angle δ shows that the phase advances
more relative to the case where the toe angles αL,
αR of the rear wheels 2L, 2R are 0°. When the rear
wheel 2 are made toed out, the follow-up of the steering torque TS
in accordance with the change in the turning angle δ shows that the
phase delays more relative to the case where the toe angles
αL, αR of the rear wheels 2L, 2R are 0°.

[0258]Here, it is assumed that the toe angle changer 120'' is in an
abnormal state. When the toe angle changer 120'' is in an abnormal state,
the rear wheel 2 is locked in any one of states among a toe-in state
(toe-in failure), a toe-out state (toe-out failure) or a state where the
rear wheels 2L, 2R are oriented in the same direction.

[0259]When the rear wheels 2L, 2R are oriented in the same direction, the
follow-up property of the steering torque TS in accordance with a
change in the turning angle δ changes, depending on the direction
of the turning angle δ of the front wheels 1. That is, when the
rear wheels 2L, 2R and the front wheels 1 are oriented in the same
direction (i.e., when the front wheels 1 and the rear wheels 2 are in the
same phase), the follow-up property of the steering torque TS in
accordance with the change in the turning angle δ becomes the same
as in the case where the rear wheels 2 are toed in (in a toe-in failure
state). On the other hand, when the rear wheels 2L, 2R and the front
wheels 1 are oriented in the opposite direction (i.e., when the front
wheels 1 and the rear wheels 2 are in antiphase), the follow-up property
of the steering torque TS in accordance with the change in the
turning angle δ becomes the same as in the case where the rear
wheels 2 are toed out (in a toe-out failure state).

[0260]As described above, when the toe angle changer 120'' is in an
abnormal state, as a toe-in failure state or a toe-out failure state, the
follow-up property of the steering torque TS in accordance with the
change in the turning angle δ, especially the damping property, is
different from those of the toe angle changer 120'' in a normal state,
and as a result, unnatural steering feeling may be given to the driver.

[0261]Specifically, when the toe angle changer 120'' is in a toe-in
failure state and the phase of the steering torque TS relative to
the turning angle δ advances, it is necessary to delay the phase of
the steering torque TS. In other words, the phase of the turning
angle δ is to be advanced.

[0262]Therefore, in the present embodiment, when the toe angle changer
120'' is in a toe-in failure state, a compensation value I is made
smaller than that in the case where the toe angle changer 120'' is in a
normal state, to thereby make the viscosity in the steering unit smaller.
Accordingly, the steering unit becomes lighter, and the phase of the
turning angle δ can be advanced.

[0263]Therefore in the present embodiment, as shown in FIG. 20, the damper
compensation signal computing part 52'' stores the damper table A 52a2
for setting a smaller compensation value I than that in the case where
the toe angle changer 120'' is in a normal state. When the changer 120''
is in a toe-in failure state, the viscosity in the steering unit can be
made smaller to obtain a suitable steering feeling, by performing a
damping control using the compensation value I set with reference to the
damper table A 52a2.

[0264]On the other hand, when the toe angle changer 120'' is in a toe-out
failure state and the phase of the steering torque TS relative to
the turning angle δ delays, it is necessary to advance the phase of
the steering torque TS. In other words, the phase of the turning
angle δ is to be delayed.

[0265]Therefore, in the present embodiment, when the toe angle changer
120'' is in a toe-out failure state, a compensation value I is made
larger than that in the case where the toe angle changer 120'' is in a
normal state, to thereby make the viscosity in the steering unit larger.
Accordingly, the steering unit becomes heavier, and the phase of the
turning angle δ can be delayed.

[0266]Therefore in the present embodiment, as shown in FIG. 20, the damper
compensation signal computing part 52'' stores the damper table B 52a3
for setting a larger compensation value I than that in the case where the
toe angle changer 120'' is in a normal state. When the toe angle changer
120'' is in a toe-out failure state, the viscosity in the steering unit
can be made larger to obtain a suitable steering feeling, by performing a
damping control using the compensation value I set with reference to the
damper table B 52a3.

[0267]In addition, in the case where the rear wheels 2L, 2R are oriented
in the same direction, and when the front wheels 1 and the rear wheels 2
are in the same phase, it has been known that the properties become the
same as those in the case where the toe angle changer 120'' is in a
toe-in failure state, as described above. On the other hand, when the
front wheels 1 and the rear wheels 2 are in antiphase, it has been known
that the properties become the same as those in the case where the toe
angle changer 120'' is in a toe-out failure state. Accordingly, when the
front wheels 1 and the rear wheels 2 are in the same phase, the
compensation value I is set in accordance with the setting property that
the increment in the compensation value I (which increases as the angular
velocity ω increases) is made smaller than that in the case where
the angle changer 120'' is in a normal state. When the front wheels 1 and
the rear wheels 2 are in antiphase, the compensation value I is set in
accordance with the setting property that the increment in the
compensation value I (which increases as the angular velocity ω
increases) is made larger than that in the case where the toe angle
changer 120'' is in a normal state.

[0268]As described above, in the present embodiment, in addition to the
damper table N 52a1 to be referred to by the steering control ECU 130''
when the toe angle changer 120'' is in a normal state, the damper
compensation signal computing part 52'' shown in FIG. 20 includes: the
damper table A 52a2 to be referred to by the steering control ECU 130''
when the toe angle changer 120'' is in a toe-in failure state; and the
damper table B 52a3 to be referred to by the steering control ECU 130''
when the toe angle changer 120'' is in a toe-out failure state. The
damping control of the steering damper is performed with switching the
damper tables in accordance with the state of the toe angle changer
120''.

[0269]Specifically, when the toe angle changer 120'' (see FIG. 20) is in a
toe-in failure state, the steering control ECU 130'' refers to the damper
table A 52a2 to set the compensation value I, and when the toe angle
changer 120'' is in a toe-out failure state, the steering control ECU
130'' refers to the damper table B 52a3 to set the compensation value I.
In this manner, by switching the damper tables to be referred to, there
can be obtained an excellent effect that, when the toe angle changer
120'' is in a toe-in failure state during vehicle turn, the viscosity in
the steering unit is made smaller, and when the toe angle changer 120''
is in a toe-out failure state during vehicle turn, the viscosity in the
steering unit is made larger.

[0270]FIG. 22 is a flow chart showing a control in which the steering
control ECU detects an anomaly of the toe angle changer and a
compensation value for damping control is set (see the drawings where
appropriate).

[0271]As shown in FIG. 22, when the steering control ECU 130'' determines
that the toe angle changer 120'' is not in an abnormal state (No at a
step S21), the steering control ECU 130'' sets a compensation value I for
damping in a normal state by referring to the damper table N 52a1 (step
S22), and the procedure returns to the step S21.

[0272]On the other hand, when the steering control ECU 130'' determines
that the toe angle changer 120'' is in an abnormal state (Yes at the step
S21), it is further determined whether or not there is a toe-in failure
state (step S23).

[0273]As described above, the toe angle change control ECU 37'' has the
self-diagnostic part 81d'', with which anomaly of the toe angle changer
120'' can be detected. Accordingly, by inputting the anomaly detection
signal from the self-diagnostic part 81d'' to the steering control ECU
130'', the steering control ECU 130'' can detect anomaly in the toe angle
changer 120''. Therefore, it can be determined whether or not the toe
angle changer 120'' is in an abnormal state.

[0274]The toe angle change control ECU 37'' can detect the toe angle
α of the rear wheel 2 by the stroke sensor 38 equipped in the
actuator 30'' of the toe angle changer 120''. Therefore, by inputting the
toe angle αL of the rear wheel 2L from the toe angle change
control ECU 37'' of the toe angle changer 120L'' to the steering control
ECU 130'', the steering control ECU 130'' can detect the toe angle
αL of the rear wheel 2L. In the same manner, the steering
control ECU 130'' can detect the toe angle αR of the rear
wheel 2R. When both of the toe angle αL of the rear wheel 2L
and the toe angle αR of the rear wheel 2R are oriented inward
relative to the vehicle (when the rear wheels are toed in), the steering
control ECU 130'' determines that the toe angle changer 120'' is in a
toe-in failure state.

[0275]In the step S23, when it is determined that the toe angle changer
120'' is in a toe-in failure state (Yes at the step S23), the steering
control ECU 130'' sets a compensation value I for damping in a toe-in
failure state by referring to the damper table A 52a2 (step S24), and the
procedure is terminated.

[0276]Referring again to the step S23, when it is determined that the toe
angle changer 120'' is not in a toe-in failure state (No at the step
S23), the steering control ECU 130'' determines whether or not the toe
angle changer 120'' is in a toe-out failure state (step S25).

[0277]Herein, when both of the toe angle αL of the rear wheel
2L and the toe angle αR of the rear wheel 2R are oriented
outward relative to the vehicle (when the rear wheels are toed out), the
steering control ECU 130'' determines that the toe angle changer 120'' is
in a toe-out failure state. When both of the toe angle αL of
the rear wheel 2L and the toe angle αR of the rear wheel 2R
are oriented in the same direction, the steering control ECU 130''
determines that the toe angle changer 120'' is not in a toe-out failure
state.

[0278]Then, in the step S25, when it is determined that the toe angle
changer 120'' is in a toe-out failure state (Yes at the step S25), the
steering control ECU 130'' sets a compensation value I for damping in a
toe-out failure state by referring to the damper table B 52a3 (step S26),
and the procedure is terminated.

[0279]Referring again to the step S25, when it is determined that the toe
angle changer 120'' is not in a toe-out failure state (No at the step
S25), the steering control ECU 130'' determines that the rear wheels 2L,
2R are oriented in the same direction, and compares the turning angle
δ of the front wheels 1 and the toe angle α of the rear
wheels 2. When it is determined that the directions of the turning angle
δ and the toe angle α are the same (Yes at a step S27), the
steering control ECU 130'' sets a compensation value I for damping in a
toe-in failure state by referring to the damper table A 52a2 (step S28);
when it is determined that the directions of the turning angle δ
and toe angle α are not the same (No at the step S27), the steering
control ECU 130'' sets a compensation value I for damping in a toe-out
failure state by referring to the damper table B 52a3 (step S29). Then,
the procedure is returned to the step S27, in order for the steering
control ECU 130'' to set a compensation value I for damper in accordance
with a change in the turning angle δ.

[0280]In this manner, when the toe angle changer 120'' is in an abnormal
state and the rear wheels 2L, 2R (see FIG. 18) are oriented in the same
direction, the damper table 52a'' to be referred to is switched as needed
in accordance with the direction of the turning angle δ of the
front wheels 1 (see FIG. 18). As a result, the compensation value for
damping can be altered, leading to an excellent effect of always
imparting a suitable steering feeling to the driver.

[0281]As described above, in the present embodiment, the damper tables to
be referred to are switched depending on the state of the toe angle
changer 120'' among a non-failure (normal) state, a toe-in failure state
and a toe-out failure state. In other words, when the toe angle changer
120'' is in a toe-in failure state, the viscosity in the steering unit is
made smaller by setting a smaller compensation value I for damping
control than usual; when in a toe-out failure state, the viscosity in the
steering unit is made larger by setting a larger compensation value I
than usual. As a result, it can be obtained an excellent effect of
imparting a natural steering feeling to the driver, even when the toe
angle changer 120'' is in an abnormal state.

Fourth Embodiment

[0282]In a fourth embodiment, a value of the base signal DT output by
the base signal computing part 51'' of the steering control ECU 130''
(see FIG. 20) is altered, when the toe angle changer 120'' (see FIG. 18)
of the steering system 100'' (see FIGS. 18 and 19) having the same
configuration as that of the third embodiment is in an abnormal state.

[0283]As described above, when the toe angle changer 120'' (see FIG. 18)
is in a toe-in failure state, the phase of the steering torque TS
relative to the turning angle δ advances. Therefore, by advancing
the phase of the turning angle δ to follow-up the phase of the
steering torque TS, a natural steering feeling can be given to the
driver. In addition, when the toe angle changer 120'' is in a toe-out
failure state, the phase of the steering torque TS relative to the
turning angle δ delays. Therefore, by delaying the phase of the
turning angle δ to follow-up the phase of the steering torque
TS, a natural steering feeling can be given to the driver.

[0284]Accordingly, in the fourth embodiment, when an anomaly is detected
in the toe angle changer, in the case where the rear wheels are in a
toe-in state, the target value of the auxiliary torque can be made
larger; while in the case where the rear wheels are in a toe-out state,
the target value of the auxiliary torque can be made smaller.

[0285]Specifically, when the toe angle changer 120'' (see FIG. 18) is in a
toe-in failure state, the steering control ECU 130'' (see FIG. 18) sets
the target value of the auxiliary torque for assisting the steering
torque TS of the electric power steering device 110'' (see FIG. 19)
to be larger. In other words, the base signal for obtaining the auxiliary
torque having the set target value is set larger. By setting the target
value of the auxiliary torque to be larger, the auxiliary torque to be
added to the steering torque TS becomes larger. Accordingly, the
steering unit becomes lighter, and the phase of the turning angle δ
can be advanced.

[0286]On the hand, when the toe angle changer 120'' is in a toe-out
failure state, the steering control ECU 130'' sets the target value of
the auxiliary torque for assisting the steering torque TS of the
electric power steering device 110'' to be small. In other words, the
base signal for obtaining the auxiliary torque having the set target
value is set smaller. By setting the target value of the auxiliary torque
to be smaller, the auxiliary torque to be added to the steering torque
TS becomes smaller. Accordingly, the steering unit becomes heavier,
and the phase of the turning angle δ can be delayed.

[0287]FIG. 23 is a schematic diagram of a control function of a steering
control ECU and toe angle changers in the steering system according to
the fourth embodiment. In FIG. 23, components which are the same as those
illustrated in the third embodiment are designated with the same
reference characters, and thus a duplicate description is omitted.

[0288]As shown in FIG. 23, in the steering control ECU 130''A according to
the fourth embodiment, in addition to a base table N 51a1 (fourth table)
and a backup table 51b, the base signal computing part 51''A further
stores a base table A 51a2 (fifth table) and a base table B 51a3 (sixth
table). The base signal computing part 51''A is configured to receive
information of anomaly of the toe angle changer 120'' from the toe angle
change control diagnostic part 73'' and to switch the tables in the base
table 51a''A to be referred to by the steering control ECU 130''A, based
on the input information. The base table A 51a2 is a table to be referred
to by the steering control ECU 130''A when the toe angle changer 120'' is
in a toe-in failure state, in which table the rate of increase in the
base signal DT in accordance with the increase in the torque signal
T is larger than that in the base table N 51a1 shown in FIG. 6A.

[0289]When the toe angle changer 120'' is in a toe-in failure state, the
steering control ECU 130''A refers to the base table A 51a2 and sets a
base signal DT corresponding to the torque signal T, to thereby set
a base signal DT larger than that in a case where the toe angle
changer 120'' is in a normal state.

[0290]The base table B 51a3 is a table to be referred to by the steering
control ECU 130''A when the toe angle changer 120'' is in a toe-out
failure state, in which table the rate of increase in the base signal
DT in accordance with the increase in the torque signal T is smaller
than that in the base table N 51a1.

[0291]When the toe angle changer 120'' is in a toe-out failure state, the
steering control ECU 130''A refers to the base table B 51a3 and sets a
base signal DT corresponding to the torque signal T, to thereby set
a base signal DT smaller than that in a case where the toe angle
changer 120'' is in a normal state.

[0292]FIG. 24 is a flow chart showing a control in which the steering
control ECU detects an anomaly of the toe angle changer and a base signal
is set (see the drawings where appropriate).

[0293]As shown in FIG. 24, when the steering control ECU 130''A determines
that the toe angle changer 120'' is not in an abnormal state (No at a
step S31), the steering control ECU 130''A sets a base signal DT for
a normal state by referring to the base table N 51a1 (step S32), and the
procedure returns to the step S31.

[0294]On the other hand, when the steering control ECU 130''A determines
that the toe angle changer 120'' is in an abnormal state (Yes at the step
S31), it is further determined whether or not there is a toe-in failure
state (step S33).

[0295]As described above, the toe angle change control ECU 37'' has the
self-diagnostic part 81d'', with which anomaly of the toe angle changer
120'' can be detected. Accordingly, by inputting the anomaly detection
signal from the self-diagnostic part 81d'' to the steering control ECU
130''A, the steering control ECU 130''A can detect anomaly in the toe
angle changer 120''. Therefore, it can be determined whether or not the
toe angle changer 120'' is in an abnormal state.

[0296]The toe angle change control ECU 37'' can detect the toe angle
α of the rear wheel 2 by the stroke sensor 38 equipped in the
actuator 30'' of the toe angle changer 120''. Therefore, by inputting the
toe angle αL of the rear wheel 2L from the toe angle change
control ECU 37'' of the toe angle changer 120L'' to the steering control
ECU 130''A, the steering control ECU 130''A can detect the toe angle
αL Of the rear wheel 2L. In the same manner, the steering
control ECU 130''A can detect the toe angle αR of the rear
wheel 2R. When both of the toe angle αL of the rear wheel 2L
and the toe angle αR of the rear wheel 2R are oriented inward
relative to the vehicle (when the rear wheels are toed in), the steering
control ECU 130''A determines that the toe angle changer 120'' is in a
toe-in failure state.

[0297]In the step S33, when it is determined that the toe angle changer
120'' is in toe-in failure state (Yes at the step S33), the steering
control ECU 130''A sets a base signal DT for a toe-in failure state
by referring to the base table A 51a2 (step S34), and the procedure is
terminated.

[0298]Referring again to the step S33, when it is determined that the toe
angle changer 120'' is not in a toe-in failure state (No at the step
S33), the steering control ECU 130''A determines whether or not the toe
angle changer 120'' is in a toe-out failure state (step S35).

[0299]Herein, when both of the toe angle αL of the rear wheel
2L and the toe angle αR of the rear wheel 2R are oriented
outward relative to the vehicle (when the rear wheels are toed out), the
steering control ECU 130''A determines that the toe angle changer 120''
is in a toe-out failure state. When both of the toe angle αL
of the rear wheel 2L and the toe angle αR of the rear wheel 2R
are oriented in the same direction, the steering control ECU 130''A
determines that the toe angle changer 120'' is not in a toe-out failure
state.

[0300]Then, in the step S35, when it is determined that the toe angle
changer 120'' is in toe-out failure state (Yes at the step S35), the
steering control ECU 130''A sets a base signal DT for a toe-out
failure state by referring to the base table B 51a3 (step S36), and the
procedure is terminated.

[0301]Referring again to the step S35, when it is determined that the toe
angle changer 120'' is not in toe-out failure state (No at the step S35),
the steering control ECU 130''A determines that the rear wheels 2L, 2R
are oriented in the same direction, and compares the turning angle
δ of the front wheels 1 and the toe angle α of the rear
wheels 2. When it is determined that the directions of the turning angle
δ and the toe angle α are the same (Yes at the step S37), the
steering control ECU 130''A sets a base signal DT for a toe-in
failure state by referring to the base table A 51a2 (step S38); when it
is determined that the directions of the turning angle δ and toe
angle α are not the same (No at the step S37), the steering control
ECU 130''A sets a base signal DT for a toe-out failure state by
referring to the base table B 51a3 (step S39). Then, the procedure is
returned to the step S37, in order for the steering control ECU 130''A to
set a base signal Din accordance with a change in the turning angle
δ.

[0302]As described above, in the fourth embodiment, the base tables to be
referred to are switched depending on the state of the toe angle changer
120'' among a non-failure (normal) state, a toe-in failure state and a
toe-out failure state. In other words, when the toe angle changer 120''
is not in an abnormal state, the steering control ECU 130''A refers to
the base table N 51a1 and sets a normal base signal DT. When the toe
angle changer 120'' is in a toe-in failure state, the steering control
ECU 130''A refers to the base table A 51a2 to set a larger base signal
DT than usual, to thereby make the auxiliary torque to be added to
the steering torque TS larger. On the other hand, when the toe angle
changer 120'' is in a toe-out failure state, the steering control ECU
130''A refers to the base table B 51a3 to set a smaller base signal
DT than usual.

[0303]In this manner, when the toe angle changer 120'' is in a toe-in
failure state, by making the auxiliary torque larger, the effect which is
equivalent to that obtained when the viscosity in the steering unit is
made smaller can be obtained; when the toe angle changer 120'' is in a
toe-out failure state, by making the auxiliary torque smaller, the effect
which is equivalent to that obtained when the viscosity in the steering
unit is made larger can be obtained. As a result, it can be obtained an
effect equivalent to the third embodiment of imparting a natural steering
feeling to the driver, even when the toe angle changer 120'' is in an
abnormal state.

[0304]When the toe angle changer 120'' is in an abnormal state and the
rear wheels 2L, 2R (see FIG. 18) are oriented in the same direction, the
tables to be referred to are switched in accordance with the direction of
the turning angle δ of the front wheels 1.

[0305]When the front wheels 1 and the rear wheels 2 are oriented in the
same direction (i.e. in the same phase), since the properties become the
same as in the case of a toe-in failure state as described above, the
steering control ECU 130''A refers to the base table A 51a2 to set the
base signal DT. On the other hand, when the front wheels 1 and the
rear wheels 2 are oriented in the opposite direction (i.e., in
antiphase), since the properties become the same as in the case of a
toe-out failure state as described above, the steering control ECU 130''A
refers to the base table B 51a3 to set the base signal DT.

[0306]In this manner, when the toe angle changer 120'' is in an abnormal
state and the rear wheels 2L, 2R (see FIG. 18) are oriented in the same
direction, the base table 51a''A to be referred to is switched as needed
in accordance with the direction of the turning angle δ of the
front wheels 1 (see FIG. 18). As a result, the target value of the
auxiliary torque can be altered, leading to an excellent effect of always
imparting a suitable steering feeling to the driver.

Fifth Embodiment

[0307]A fifth embodiment of the present invention will be described with
reference to FIGS. 3, 12, 15, 16 and 25-30. It should be noted that
components which are the same as those illustrated in the above-mentioned
embodiments are designated with the same reference characters, and thus a
duplicate description is omitted, and that components which are
equivalent, corresponding or similar to those illustrated in the
above-mentioned embodiments, are designated with the same reference
characters with a triple prime ('''), and will be described in detail
when necessary.

[0308]FIG. 25 is a schematic diagram of a four-wheel vehicle having a
steering system according to a fifth embodiment of the present invention.
In the description of the steering system according to the fifth
embodiment, an actuator of the power steering device is represented by an
electric power steering device using an electric motor for the
illustration purpose.

1. Configuration of Steering System

[0309]As shown in FIG. 25, a steering system 100''' of the fifth
embodiment includes an electric power steering device 110''', toe angle
changers 120L''', 120R''', a steering controller (hereinbelow, referred
to as "steering control ECU") 130''', and various sensors including a
front wheel turning angle sensor Ss and a vehicle speed sensor SV.
Other components are substantially the same as in the steering system
according to the first embodiment shown in FIG. 1, and when different,
the component will be described.

2. Configuration of Electric Power Steering Device

[0310]The electric power steering device 110''' of the present embodiment
is substantially the same as the electric power steering device of the
second embodiment shown in FIG. 11, except that the electric power
steering device 110''' includes steering control ECUs 130'''.

[0311]The electric motor steering control ECU 130''' of the steering
system 100''' has an electric power steering control part 130a''' (which
will be described below; see FIG. 26) as a functional part of the
electric power steering device 110''', which controls the driving of the
electric motor 4.

3. Configuration of Toe Angle Changer

[0312]Next, a configuration of the toe angle changer will be described.

[0313]It should be noted that the toe angle changer 120L''' in the present
embodiment is different from the toe angle changer 120L according to the
first embodiment shown in FIG. 3, in that the actuator 30, the toe angle
change control ECU 37, and the steering control ECU 130 in the first
embodiment are replaced with an actuator 30''', a toe angle change
control ECU 37''' and the steering control ECU 130''', respectively. The
other components are the same as those in the first embodiment and thus a
duplicate description is omitted. The configurational arrangement of all
components including the corresponding components is the same as that in
the first embodiment (see FIG. 3), and thus the duplicate drawing and the
duplicate description are omitted.

[0314]The actuator 30''' in the present embodiment is the same as the
actuator 30' in the second embodiment shown in FIG. 12, and thus the
duplicate drawing and the duplicate description are omitted.

4. Functional Configuration of Steering Control ECU

[0315]Next, functions of the steering control ECU will be described with
reference to FIG. 26.

[0316]FIG. 26 is a schematic diagram of a control function of a steering
control ECU and toe angle changers in the steering system according to a
fifth embodiment.

[0317]As shown in FIG. 26, the steering control ECU 130''' includes: the
electric power steering control part 130a''' configured to control the
electric power steering device 110''' (see FIGS. 25 and 2); and a rear
wheel toe angle control part 130b''' configured to compute target toe
angles of the rear wheels 2L, 2R.

(Electric Power Steering Control Part)

[0318]With respect to the electric power steering control part 130a''',
only the portions different from those of the second embodiment, and
configurations associated therewith, will be described with reference to
FIG. 26.

[0319]The output signal IM1 of the adder 62 is a target signal for
the Q-axis current which defines the torque of the electric motor 4, and
input to an anomaly auxiliary torque restriction part 263, which
restricts the auxiliary torque when the electric power steering device is
in an abnormal state.

[0320]The anomaly auxiliary torque restriction part 263 is configured to
suppress the input signal IM1 to less than a specific value, when an
anomaly detection signal of a high winding temperature mode from a
self-diagnostic part 272, which will be described below, is detected and
the input signal IM1 exceeds the specific value.

[0321]The anomaly auxiliary torque restriction part 263 is configured to
reduce with time the signal IM1 from the adder 62 at the moment a
correction command signal is input from a toe angle change control
diagnostic part 73''' (which will be described below), by a specific
correction amount ΔIM as shown in FIG. 14 and to output the output
signal IM2 to the adder 64.

[0322]The magnitude of the correction amount ΔIM is set to be a
specific percent, such as 80%, of the signal IM1. The current
changes between positive and negative when the steering wheel is turned
left and right, respectively, but regardless of the sign (positive or
negative) of the signal IM1, by diminishing the signal IM1 to
80% of the input value, the auxiliary torque can be reduced in such a
manner that the driver can sense the reduction.

[0323]When the correction command signal is not input from the toe angle
change control diagnostic part 73''', the anomaly auxiliary torque
restriction part 263 outputs the signal IM1 as-is as an output
signal IM2 to the adder 64.

(Rear Wheel Toe Angle Control Part)

[0324]Next, with respect to the rear wheel toe angle control part 130b''',
only the portions different from those of the second embodiment, and
configurations associated therewith, will be described with reference to
FIG. 26. A shown in FIG. 26, the rear wheel toe angle control part
130b''' includes a target toe angle computing part 71''', the
self-diagnostic part 272 and the toe angle change control diagnostic part
73'''.

[0325]The front wheel turning angle sensor Ss is configured to detect
(measure) a turning angle δ of the front wheels 1L, 1R and to input
the result to the target toe angle computing part 71'''.

[0326]The target toe angle computing part 71''' is configured to generate
target toe angles αTL, αTR for the rear wheels 2L,
2R, respectively, based on the vehicle speed signal VS, a turning angle
δ, and a turning angular velocity δ' which is easily obtained
by temporal differentiation of the turning angle δ, and to input
the target toe angles αTL, αTR to the respective
toe angle change control ECUs 37''', 37''' configured to control
respective toe angle changes of the left rear wheel 2L and the right rear
wheel 2R (see FIG. 15). The target toe angles αTL,
αTR are generated from a first toe angle table 71a''' and a
second toe angle table 71b''', with reference to the turning angle
δ, the turning angular velocity δ' and the vehicle speed V,
which tables had been prepared for each of the left rear wheel 2L and the
right rear wheel 2R in advance.

[0327]The first toe angle table 71a''' is a table to be referred when the
electric power steering device 110''' is in a normal state. The second
toe angle table 71b''' is a table to be referred when the auxiliary
torque function of the electric power steering device 110''' is in an
abnormal state and an anomaly detection signal indicating a failure mode
is received from the self-diagnostic part 272, which will be described
later.

[0328]In the first toe angle table 71a''', the target toe angles
αTL, αTR of the rear wheels are set in a similar
manner to that of the second embodiment (thus a detailed description is
omitted here).

[0329]Also in the second toe angle table 71b''', the target toe angles
αTL, αTR are generated in the same manner as in the
second embodiment described with the formulae (6) and (7). However, in
order to prevent discomfort to the driver due to a mismatch between the
steering feeling to the driver and the vehicle turnability, which may
otherwise be caused when the auxiliary torque function of the electric
power steering device 110''' is in an abnormal state and the steerability
is reduced, the target toe angle controls which are allowed in the first
toe angle table 71a''' are prohibited. Specifically, in the high-speed
range over the above-mentioned specific low-speed range, when an absolute
value of the turning angular velocity δ' is a specific value or
less, and at the same time, the turning angle δ is within a
specific range (including right and left), it is prohibited that the
target toe angle control be performed in which the target toe angles
αTL, αTR of the rear wheels 2L, 2R are set as the
same phase relative to the front wheels, in accordance with the turning
angle δ. In the high-speed range over the above-mentioned specific
low-speed range, when the absolute value of the turning angular velocity
δ' exceeds a specific value, or when the turning angle δ is
too large to fall outside the specific range (including right and left),
it is prohibited that the target toe angle control be performed in which
the target toe angle αTL, αTR of the rear wheels
are set to antiphase relative to the front wheels, in accordance with
turning angle δ. To sum up, the data in the second toe angle table
71b''' is set in advance in such a manner that, when the vehicle speed V
is in the high-speed range over the above-mentioned specific low-speed
range, at least the target toe angle αTL of the left rear
wheel 2L and the target toe angle αTR of the right rear wheel
2R do not exceed 0° (do not become toe-out), for any turning angle
δ and turning angular velocity δ'.

[0330]The specific low-speed range herein means a range from no running
state to the lowest speed of, for example, 10 km/hr.

[0331]Next, the self-diagnostic part (anomaly detection unit) 272 will be
described with reference to FIG. 26. The self-diagnostic part 272 is
configured to determine whether or not an abnormal state of the electric
power steering device 110''' is detected, based on an angular signal
θ from the resolver 25, a detection signal from a Hall element (not
shown) of the electric motor drive circuit 23, a temperature signal from
a sensor 4a provided in the electric motor 4, and a state monitoring of
the output signal from the adder 64.

[0332]For example, the self-diagnostic part 272 determines that a winding
temperature of the electric motor 4 is in an abnormal state (high winding
temperature mode) when the signal from the temperature sensor 4a exceeds
a specific value, and outputs an anomaly detection signal indicating a
high winding temperature mode to the anomaly auxiliary torque restriction
part 263 to suppress the target current.

[0333]The self-diagnostic part 272 is configured to monitor the output
signal IE from the adder 64 and the detection signal of actual current
from a Hall element of the electric motor drive circuit 23; to determine
whether or not the auxiliary torque function of the electric power
steering device 110''' is in an abnormal state, based on the angular
signal θ from the resolver 25. The self-diagnostic part 272 is also
configured, when it is determined that the auxiliary torque function is
in an abnormal state (failure mode), to command the electric motor drive
circuit 23 to stop the power supply to the electric motor 4; to output an
anomaly detection signal indicating the failure mode to the target toe
angle computing part 71'''; and to switch the tables to be referred to by
the target toe angle computing part 71''' for computing the target toe
angles αTL, αTR to be output to the toe angle
change control ECU 37''' of the toe angle changers 120L''', 120R''', from
the first toe angle table 71a''' to the second toe angle table 71b'''.

[0334]The toe angle change control diagnostic part 73''' is similar to the
toe angle change control diagnostic part 73' in the second embodiment,
and thus a duplicate description is omitted.

(Toe Angle Change Control ECU)

[0335]The configuration of the toe angle change control ECU 37''' in the
present embodiment is substantially the same as that of the toe angle
change control ECU 37' in the second embodiment shown in FIG. 15, and
thus the duplicate drawing and the duplicate description are omitted.

[0336]Next, with reference to FIG. 27, a flow of controlling the toe angle
change of the rear wheels, when the electric power steering device 110'''
has an anomaly in auxiliary torque, will be described. FIG. 27 is a flow
chart showing a control in the target toe angle computing part. This
control process is performed in a predetermined cycle.

[0337]In a step S41, immediately after initiating the steering control ECU
130''', IFLAG=0 indicating a normal state is set as an initial setting.
Subsequently in a step S42, it is determined whether or not the auxiliary
torque function of the electric power steering device 110''' is in an
abnormal state (EPS (Electric Power Steering) failure), based on whether
or not the anomaly detection signal indicating a failure mode as
described above is received from the self-diagnostic part 272. When it is
determined that the electric power steering device 110''' is not in an
EPS failure state (No), the procedure advances to a step S43, and in an
EPS failure state (Yes), the procedure advances to a step S44.

[0338]In the step S43, the target toe angles αTL,
αTR are calculated by referring to the first toe angle table
71a''' in accordance with the vehicle speed signal VS, the turning angle
δ and the turning angular velocity δ', and the results are
output to the toe angle change control ECU 37''' of the respective toe
angle changers 120L''', 120R''' (normal RTC control). The procedure
returns to the step S42, and a series of the control is repeated.

[0339]The normal RTC control performs the control of the toe angles of the
rear wheels 2L, 2R to the same phase or to antiphase in accordance with
the vehicle speed signal VS, the turning angle δ and the turning
angular velocity δ', to thereby allow the vehicle to turn in a
small radius at a low speed, to quickly change lanes at a high speed, and
to quickly turn for risk aversion at a high speed.

[0340]In the step S44, it is determined that the condition IFLAG=0 is met
or not. When it is determined that IFLAG=0 is satisfied (Yes), the
procedure advances to a step S45, and that IFLAG=0 is not satisfied (No),
the procedure advances to a step S47. In the step S45, the first toe
angle table is switched to the second toe angle table. Then, IFLAG=1
indicating the auxiliary torque anomaly mode is set (step S46), and the
procedure advances to the step S47.

[0341]In the step S47, the target toe angles αTL,
αTR are calculated by referring to the second toe angle table
71b''' in accordance with the vehicle speed signal VS, the turning angle
δ and the turning angular velocity δ', and the results are
output to the toe angle change control ECU 37''' of the respective toe
angle changers 120L''', 120R''' (anomaly RTC control). The procedure
returns to the step S42, and a series of the control is repeated.

[0342]In an anomaly RTC control, when the vehicle runs at a sufficiently
low speed, the toe angles of the rear wheels 2L, 2R are allowed to become
antiphase, so as to allow the vehicle to turn in a small radius. However,
when the vehicle runs at a high speed, the steerability becomes low due
to the EPS failure, and in order to correspond to this state, a priority
is given to securing a stable turnability, and thus the toe-out control
of the rear wheels 2L, 2R is not allowed.

[0343]The control flow in the toe angle change control diagnostic part
73''' is substantially the same as that in the second embodiment shown in
FIG. 16, and thus the duplicate drawing and the duplicate description are
omitted.

[0344]As described above, according to the present embodiment, when the
target toe angle computing part 71''' receives, from the self-diagnostic
part 272, an anomaly detection signal indicating a failure mode in which
it is determined that there is an anomaly in the auxiliary torque
function of the electric power steering device 110''' (i.e., it is
determined that the auxiliary torque cannot be output), in the case of
the high-speed range over the above-mentioned specific low-speed range
of, for example, 10 km/hr, the steering control ECU 130''' switches to
the control in which the rear wheels 2L, 2R are not allowed to be toed
out (when the anomaly detection unit detects an abnormal state of the
power steering device, the toe angle changer is controlled so that at
least toe-out control which improves turnability is not allowed).
Therefore, in accordance with the notably reduced steerability due to no
auxiliary torque and thus to a large reaction force for steering the
steering wheel 3, a light turnability by the rear wheel steering is not
imparted (discomfort feeling is prevented which may otherwise be caused
by less reduction in turnability than the reduction expected from the
heavy steering feeling during running), leading to a stable turn
operation provided to the driver.

[0345]As a result, discomfort feeling can be prevented which may otherwise
be caused by the same-phase control of the rear wheels 2L, 2R during
high-speed driving with no auxiliary torque in the electric power
steering device 110'''.

[0346]When the toe angle change control diagnostic part 73''' receives an
anomaly detection signal from the toe angle change control ECU 37''', and
when both of the rear wheels 2L, 2R are not in a toe-in state or when the
actual toe angles αSL=αSR=0 is not satisfied, or
when it is not the case that the one of the rear wheels is in a toe-in
state and the actual toe angle of the other rear wheel is zero, the
steering control ECU 130''' makes the adder 64 output in such a manner
that the auxiliary torque is reduced, after confirming that the vehicle
speed is Vlow or lower. With this configuration, the auxiliary
torque becomes smaller at a sufficiently low speed, and the responsive
feeling from the steering torque given to the driver becomes large,
making it easier for the driver to sense an abnormal state of the
steering function. Therefore, an awkward feeling, which may otherwise be
given to the driver as a result of a sudden reduction in the auxiliary
torque during running, can be prevented. Further, even when the steering
feeling is heavy, an excellent turnability can be maintained, and for
example, the vehicle can turn in a small radius during a parking
operation of the vehicle.

[0347]Since the gain is set in such a manner that the auxiliary torque is
reduced in accordance with the increase in the speed, if the vehicle runs
at a vehicle speed exceeding the vehicle speed Vlow, an anomaly in
the toe angle changers 120L''' and/or 120R''' is easily sensed by the
driver. Therefore, the driver can easily reduce the vehicle speed in
response to the discomfort feeling in steerability, and when the vehicle
speed reaches Vlow or lower, the auxiliary torque is reduced, to
thereby surely inform the driver of anomaly in the toe angle changers
120L''' and/or 120R'''.

[0348]In this case, the auxiliary torque is reduced with a time delay, and
thus it is possible to prevent the driver from being surprised at a rapid
decrease in the auxiliary torque.

[0349]It should be noted that, the reason for not reducing the auxiliary
torque when the signal from the toe angle change control ECU 37'''
indicates that the rear wheels 2L, 2R are in a toe-in state or
αSL=αSR=0 is satisfied is that, when the rear
wheels 2L, 2R are locked in the toe-in state or locked with
αSL=αSR=0 steerability and turnability are the same
as those of a conventional vehicle with the steering of only front wheels
1L, 1R. Therefore, it would suffice to warn the driver by turning on an
alarm lamp provided on a console, which informs anomaly in the toe angle
changers 120L''' and/or 120R'''.

[0350]When the self-diagnostic part 81d''' of one of the toe angle change
control ECUs 37''', 37''' detects an abnormal state, the self-diagnostic
part 81d''' sends an anomaly detection signal to the other toe angle
change control ECU 37''', and both of the toe angle changers 120L''',
120R''' are controlled so that both of the toe angles are fixed.
Therefore, it is prevented that a change of only one of the toe angles
between the rear wheels 2L, 2R remains controlled, and thus a driving
performance is maintained stable even when the toe angle changers
120L''', 120R''' are in an abnormal state.

(First Modified Version of Fifth Embodiment)

[0351]Next, a steering system 100'''A according to a first modified
version of the fifth embodiment will be described with reference to FIGS.
25, 28 and 29. FIG. 28 is a schematic diagram of a control function of a
steering control ECU and toe angle changers in the steering system
according to a first modified version of the fifth embodiment. FIG. 29 is
a detailed diagram of a control function of a target toe angle computing
part of the steering control ECU. Components which are the same as those
illustrated in the fifth embodiment are designated with the same
reference characters, and thus a duplicate description is omitted.

[0352]It should be noted that the steering system 100'''A in the present
modified version is different from the steering system 100''' according
to the fifth embodiment shown in FIG. 25, in that the steering control
ECU 130''' in the first embodiment is replaced with the steering control
ECU 130'''A. The other components are the same as those in the fifth
embodiment and thus a duplicate description is omitted. The
configurational arrangement of all components including the corresponding
components is the same as that in the fifth embodiment (see FIG. 25), and
thus the duplicate drawing and the duplicate description are omitted.

[0353]The steering system 100'''A according to the present modified
version has a yaw rate sensor SY, as shown in FIG. 25, which is
configured to detect (measure) an actual yaw rate of the vehicle body and
to output the result to a steering control ECU 130'''A.

[0354]The steering control ECU 130'''A has, as shown in FIG. 28, a target
toe angle computing part 71'''A, instead of the target toe angle
computing part 71''' of the fifth embodiment.

[0355]As shown in FIG. 29, the target toe angle computing part 71'''A
includes: a feedforward control part 90a having a turning angle
feedforward part (hereinbelow, simply referred to as "turning angle F/F
part") 91, a turning angular velocity computing part 92, a turning
angular velocity feedforward part (hereinbelow, referred to as "turning
angular velocity F/F part") 93 and an adder 94; a feedback control part
90b having a reference yaw rate computing part 95, a subtracter 96, a yaw
rate feedback part (hereinbelow, simply referred to as "yaw rate F/B
part") 97; and an adder 98.

[0356]The turning angle F/F part 91 is configured to receive signals of
the turning angle δ and vehicle speed V and an anomaly detection
signal from the self-diagnostic part 272, and to calculate feedforward
output values α1L, α1R of the target toe angle by
referring to a first gain table 91a or a second gain table 91b. These
gain tables 91a, 91b are look-up tables stored in a memory in advance,
for calculating the feedforward output values α1L,
α1R using a turning angle δ and a vehicle speed V as
parameters.

[0357]The turning angle F/F part 91 is configured to refer to the first
gain table 91a when in a normal state, to control the ratio of the
feedforward based on the vehicle speed V and the turning angle δ,
and to set absolute values of the feedforward output values
α1L, α1R of the target toe angle larger, when the
turning angle δ is large.

[0358]The turning angle F/F part 91 is configured to switch from the first
gain table 91a to the second gain table 91b when it receives an anomaly
detection signal indicating a failure mode, and to calculate feedforward
output values α1L, α1R by referring to the second
gain table 91b.

[0359]In the calculation of the target toe angles in the turning angle F/F
part 91, the left and right directions of the rear wheels 2L, 2R are
defined as positive and negative, respectively. When the turning angle
F/F part 91 refers to the first gain table 91a to calculate the target
toe angle, in the case where the vehicle speed V is in the specific
low-speed range as described above, the feedforward output values
α1L, α1R of the target toe angle are set in such a
manner that the rear wheels 2L, 2R are in antiphase relative to the
turning angle δ, and thus to allow the vehicle to turn in a small
radius. In the high-speed range over the above-mentioned specific
low-speed range, when the turning angle δ is within a specific
range (including right and left), the turning angle F/F part 91 refers to
the first gain table 91a and sets the output values α1L,
α1R in such a manner that the rear wheels 2L, 2R are in the
same phase relative to the turning angle δ. In the high-speed range
over the above-mentioned specific low-speed range, when the turning angle
δ is too large to fall outside the specific range (including right
and left), the turning angle F/F part 91 refers to the first gain table
91a and sets the output values α1L, α1R in such a
manner that the rear wheels 2L, 2R are in antiphase relative to the
turning angle δ.

[0360]Unlike the first gain table 91a, the second gain table 91b is set so
that the feedforward output values α1L, α1R are
under toe-in control.

[0361]The turning angular velocity computing part 92 is configured to
calculate a turning angular velocity δ' by temporal differentiation
of the turning angle δ, and to input the result to the turning
angular velocity F/F part 93.

[0362]The turning angular velocity F/F part 93 is configured to receive
signals of the turning angular velocity δ' and vehicle speed V and
an anomaly detection signal from the self-diagnostic part 272, and to
calculate feedforward output values α2L, α2R of the
target toe angle by referring to a first gain table 93a or a second gain
table 93b. These gain tables 93a, 93b are look-up tables stored in a
memory in advance, for calculating feedforward output values
α2L, α2R using a turning angular velocity δ'
and a vehicle speed V as parameters.

[0363]The turning angular velocity F/F part 93 is configured to refer to
the first gain table 93a when in a normal state, to control the ratio of
the feedforward based on the vehicle speed V and the turning angular
velocity δ', and to set absolute values of the feedforward output
values α2L, α2R larger, when the turning angular
velocity δ' is large.

[0364]The turning angular velocity F/F part 93 is configured to switch
from the first gain table 93a to the second gain table 93b when it
receives an anomaly detection signal indicating a failure mode, and to
calculate feedforward output values α2L, α2R, by
referring to the second gain table 93b. The gain in the second gain table
93b is set in such a manner that the feedforward output values
α2L, α2R become smaller as compared with the first
gain table 93b.

[0365]As a result, when in a normal state, the turning angular velocity
F/F part 93 sets the feedforward output values α2L,
α2R in such a manner that a light turning motion of the
vehicle is realized in accordance with the turning angular velocity
δ'; when receiving an anomaly detection signal indicating a failure
mode, the turning angular velocity F/F part 93 sets the feedforward
output values α2L, α2R in such a manner that the
turning motion of the vehicle does not quickly respond to the turning
angular velocity δ'.

[0366]The feedforward output values α1L, α1R of the
target toe angle calculated in the turning angle F/F part 91 and the
feedforward output values α2L, α2R calculated in
the turning angular velocity F/F part 93 are added in the adder 94, which
gives the feedforward output values α3L
(=α1L+α2L), α3R
(=α1R+α2R) of the target toe angle, to be input to
the adder 98 and at the same time to the reference yaw rate computing
part 95.

[0367]To the reference yaw rate computing part 95, the signals of turning
angle δ and vehicle speed V, the anomaly detection signal from the
self-diagnostic part 272 and the feedforward output values
α3L, α3R of the target toe angle are input, and the
reference yaw rate computing part 95 calculates a reference yaw rate
γm by referring to a first reference yaw rate table 95a or a
second reference yaw rate table 95b. The first reference yaw rate table
95a is referred to when the electric power steering device 110''' is in a
normal state, and the second reference yaw rate table 95b is referred to
when auxiliary torque function of the electric power steering device
110''' is in an abnormal state. These reference yaw rate tables 95a, 95b
are look-up tables stored in a memory in advance, for computing the
reference yaw rate γm using, for example, the vehicle speed
signal VS, the turning angle δ, the output values α3L,
α3R of the target toe angle as parameters.

[0368]The first reference yaw rate table 95a is set in advance so as to
calculate a reference yaw rate γm as a target standard yaw
rate during expected light turning motion, by setting the toe angles of
the rear wheels to antiphase or the same phase relative to the turning
angle δ, based on the combination of the vehicle speed V and the
turning angle δ.

[0369]As shown in FIG. 29, upon receiving an anomaly detection signal
indicating a failure mode from the self-diagnostic part 272, the
reference yaw rate computing part 95 switches from the first reference
yaw rate table 95a to the second reference yaw rate table 95b. The second
reference yaw rate table 95b is set in advance so as to calculate a
reference yaw rate γm which is expected to provide a stable
turning behavior with the turning angle δ being valued.

[0370]The reference yaw rate γm calculated in the reference yaw
rate computing part 95 is put in the subtracter 96, and a deviation
Δγ between the reference yaw rate γm and an actual
yaw rate γ from the yaw rate sensor SY is obtained, which is
input to the yaw rate F/B part 97.

[0371]The yaw rate F/B part 97 in a normal state refers to a first gain
table 97a in accordance with the actual yaw rate γ and the
deviation Δγ, and outputs feedback output values
α4L, α4R of the target toe angle to the adder 98.
Then, as shown in FIG. 29, to the yaw rate F/B part 97, an anomaly
detection signal from the self-diagnostic part 272 is input, and when an
anomaly detection signal indicating a failure mode is received, the first
gain table 97a is switched to a second gain table 97b. When an anomaly
detection signal indicating a failure mode is received, the yaw rate F/B
part 97 refers to the second gain table 97b in accordance with the actual
yaw rate γ and the deviation Δγ to thereby output the
feedback output values α4L, α4R of the target toe
angle to the adder 98.

[0372]These gain tables 97a, 97b are look-up tables stored in a memory in
advance, for calculating feedback output values α4L,
α4R using a yaw rate and a deviation Δγ as
parameters.

[0373]The first gain table 97a is set in such a manner that the gain is
set to make the feedback larger relative to the deviation Δγ
of the yaw rate. On the other hand, the second gain table 97b is set in
such a manner that the gain is set to make the feedback smaller relative
to the deviation Δγ of the yaw rate, as compared with the
first gain table 97a.

[0374]In the adder 98, the feedforward output values α3L,
α3R of the target toe angle and the feedback output values
α4L, α4R of the target toe angle are added,
respectively, and the target toe angles αTL
(=α3L+α4L), αTR
(=α3L+α4L) are input to the respective toe angle
change control ECUs 37''', 37''' of the left rear wheel 2L and the right
rear wheel 2R.

[0375]Herein, the first gain tables 91a, 93a, 97a and the first reference
yaw rate table 95a form a standard vehicle model in a normal state, while
the second gain tables 91b, 93b, 97b and the second reference yaw rate
table 95b form a standard vehicle model in an abnormal state of a failure
mode.

[0376]In the target toe angle computing part 71'''A, switching from the
standard vehicle model in a normal state to the standard vehicle model in
an abnormal state of a failure mode is executed in the same manner as in
the control flow chart (FIG. 27) of the target toe angle computing part
71''' (see FIG. 15) according to the fifth embodiment, while the steering
control ECU 130''' is replaced with the steering control ECU 130'''A, the
target toe angle computing part 71''' is replaced with the target toe
angle computing part 71'''A, and the first toe angle table 71a''' and the
second toe angle table 71b''' are replaced with the standard vehicle
model in a normal state and the standard vehicle model in an abnormal
state of a failure mode, respectively. The description "switching tables"
of the step S45 in the flow chart in FIG. 27 is replaced with "switching
standard vehicle models".

[0377]FIG. 30 show a comparison in yaw rate response property in a turning
motion between a standard vehicle model according to the first modified
version of the fifth embodiment and a vehicle having only a front wheel
steering (hereinbelow, simply referred to as "conventional vehicle") in
which: FIG. 30A is a graph showing yaw rate response property with
steering frequency on the horizontal axis and yaw rate gain on the
vertical axis. FIG. 30B is a graph showing a yaw rate response property
with steering frequency on the horizontal axis and yaw rate phase on the
vertical axis.

[0378]The standard vehicle model (normal state) according to the first
modified version of the fifth embodiment is set in such a manner that, as
indicated with a broken line in FIG. 30A, the resonant frequency is
enhanced in an right arrow direction and the resonance gain is reduced in
a down-arrow direction to thereby increase an attenuation rate, as
compared with the conventional vehicle indicated with a dashed-dotted
line; and set in such a manner that, as indicated with a broken line in
FIG. 30B, the phase delay of the yaw rate is reduced in an up-arrow
direction, as compared with the conventional vehicle indicated with a
dashed-dotted line.

[0379]With this setting, in a normal state, a vehicle with a high response
is obtained that can follow up a quick steering by the driver.

[0380]The standard vehicle model (abnormal state of a failure mode)
according to the first modified version of the fifth embodiment is set in
such a manner that, as indicated with a solid line in FIG. 30A, the yaw
rate gain of the standard vehicle model is made lower than that of the
standard vehicle model in a normal state; and also set in such a manner
that, as shown in FIG. 30B, the phase delay of the yaw rate is made same
as that of the standard vehicle model in a normal state.

[0381]With this setting, in an abnormal state of auxiliary torque in the
electric power steering device 110''', the standard vehicle model is
switched to the vehicle with a low response, to thereby surely provide
the stability of the vehicle.

[0382]It should be noted that, in the fifth embodiment and the first
modified version thereof, it is preferable that the switching from the
target toe angle control for a normal state to that for an abnormal state
of a failure mode is performed when the vehicle is not in turning motion
or when the vehicle runs at a sufficiently low speed, since an awkward
feeling, which may otherwise be given to the driver as a result of a
sudden change in response property of vehicle during turning motion, can
be prevented.

(Other Modified Versions of Fifth Embodiment)

[0383]The present invention is not limited to the fifth embodiment and the
first modified version thereof, and it is possible to make, for example,
the following various modifications.

(1) In the electric power steering control part 130a''' according to the
fifth embodiment and the first modified version thereof, the current in
the electric motor 4 is controlled by setting the target current.
Instead, a target voltage may be set as a voltage to be applied to the
electric motor 4. Alternatively, a target torque may be set as a torque
to be output by the electric motor 4, to thereby control the current in
the electric motor 4. Such a target voltage and a target torque are
included in the target signal.(2) In the fifth embodiment and the first
modified version thereof, as shown in FIG. 6A, the base signal computing
part 51 generates the base signal DT to be used as a standard
reference for a target signal IM1 for the output torque TM*,
based on the torque signal T and the vehicle speed signal VS as a
parameter, but the present invention is not limited to these embodiments.
The present invention can be applied to an electric power steering device
in which: an operation angle signal from an operation angle sensor
provided in the steering wheel 3 and a yaw rate signal from the yaw rate
sensor SY are input to the steering control ECU 130''' or the
steering control ECU 130'''A, the reference yaw rate, which is determined
in advance based on the vehicle speed signal VS and the operation angle,
is calculated; and feedback control of the reaction force to the steering
wheel 3 is performed based on a difference between the reference yaw rate
and the actual yaw rate.

[0384]The present invention can be applied to an electric power steering
device in which: a yaw rate signal from the yaw rate sensor SY is
input to the steering control ECU 130''' or the steering control ECU
130'''A; and a yaw rate feedback reaction force torque, which is
calculated based on the vehicle speed signal VS and the yaw rate, is fed
back as a reaction force to the steering wheel 3.

[0385]In the steering system according to the fifth embodiment and the
first modified version thereof, when the electric power steering device
is in an abnormal auxiliary torque state, it is prohibited to perform the
control that makes the toe angles αL, αR of the
rear wheels toed out. This means that the toe angle of the rear wheels
are changed to the direction that suppresses the generation of yaw rate.
The steering system in which the left and right toe angles of the rear
wheels are controlled to become the same phase includes those in which it
is prohibited that a rear wheel as an outer ring of turning becomes
toe-out, i.e., antiphase of the left and right toe angles of the rear
wheels relative to the turning angle δ of the front wheels.